Interlingua is a planned language. It uses words that are found in most West-European languages. It was made by IALA. They are a group of people (the most known was Alexander Gode) that worked on it for more than 20 years. The first dictionary for the language was finished and published in 1951. Interlingua was created on the base of languages: English, French, Spanish, Portuguese, and Italian.

Spelling 
The vowels in Interlingua are pronounced like in Spanish or LFN (another planned language). The consonants are pronounced, for the most part, like in English or LFN.

Interlingua keeps much of the spelling of Latin words, more near to the forms in English and French than to them in the other Romance languages. For example, it keeps q ([k]), y ([i]), ch ([k]), ph ([f]), rh ([r]), and th ([t]). It keeps also the common use of double consonants. More, it allows versions like ch for [ʃ], s between vowels like [z], c before e/i like [ts], g before e/i like [ʒ], qu before e/i like [k], etc.

Accent is not indicated, even in exceptions, but it follows, for the most part, a system of rules similar to that of LFN.

Collateral spelling 
Interlingua has also a "collateral" spelling, that simplifies the complications above to a system more like this of Spanish or LFN. This collateral spelling has the same status, but in reality it is not much used by adherents. The details:

double letters are simplified, except ss, for example ecclesia > eclesia, adducer > aducer, interrogar > interogar.
vowel y becomes i, for example tyranno > tirano.
ph becomes f, for example phonetic > fonetic.
ch, when pronounced like k, becomes c, except before e o i, for example christo > cristo.
rh and th become r and t, for example rhetoric > retoric, pathetic > patetic.
g and gi, when pronounced like French j, become j, for example sage > saje, sagio > sajo.
-isar and derivations become -izar, for example civilisar > civilizar.
-te becomes -t, except when the accent is on the third syllable from the end, for example animate > animat.
-nne, -lle, and -rre become -n, -l, and -r, for example perenne > peren, belle > bel, bizarre > bizar.

Articles 
un - a/an
le - the
al - to the
del - of the

Nouns 
The plural is -s after a vowel, -es after a consonant, but -hes after final c:
catto > cattos - cat > cats
can > canes - dog > dogs
roc > roches - rook > rooks (of chess)

Interlingua has no grammatical gender. Some words distinguish female from male by changing -o into -a, or by adding -essa. Other ones have two different forms. But the most of words do not distinguish:
puero > puera - boy > girl
tigre > tigressa - male tiger > female tiger
rege > regina - king > queen
jornalista - journalist (male or female)

Adjectives 
Adjectives do not change for agreement with nouns. They can precede or follow the noun, except numbers, that always precede the noun. In general, short adjectives precede and long adjectives follow. 
belle oculos = oculos belle - beautiful eyes
un bon idea, un idea ingeniose - a good idea, an ingenious idea

To compare, use plus or minus and le plus or le minus:
un plus feroce leon - a more ferocious lion
un traino minus rapide - a less rapid train
le plus alte arbore - the highest tree
le solution le minus costose - the less costly solution

One can use the suffix -issime for the absolute superlative:
un aventura excellentissime -  the most excellent adventure

The adjectives bon, mal, magne (big), and parve (little) have alternative irregular forms :

{|
|-
|bon › plus bon › le plus bon ||   || or ||   || bon › melior › optime
|-
|mal › plus mal › le plus mal ||   || or ||   || mal › pejor › pessime
|-
|magne › plus magne › le plus magne ||   || or ||   || magne › major › maxime
|-
|parve › plus parve › le plus parve ||   || or ||   || parve › minor › minime
|}

Adverbs 
There are two kinds of adverbs: first form and second form. Adverbs of first form are a closed class of grammatical words, like quasi (almost), jam (already), and totevia (nevertheless). Adverbs of second form are an open class derived from adjectives by adding the suffix -mente (or -amente after final c):
felice > felicemente - happily
magic > magicamente - magically

Some common adverbs have short alternative forms that end with -o:
sol > solmente > solo - only

Like adjectives, adverbs use plus, minus, le plus, and le minus:
Illa canta plus bellemente que illa parla - She sings more beautifully than she speaks
Le gepardo curre le plus rapide de omne animales - The cheetah runs the most rapidly of all animals

The adverbs of bon and mal have alternative irregular forms:

{|
|-
|bonmente › plus bonmente › le plus bonmente ||   || or ||   || ben › plus ben › le plus ben ||   || or ||   || ben › melio › optimo
|-
|malmente › plus malmente› le plus malmente ||   || or ||   || mal › plus mal › le plus mal ||   || or ||   || mal › pejo › pessimo
|}

Pronouns

Personal pronouns 

One can use the objective form for the direct object and the indirect object. Or, one can use the preposition a before the pronoun for the indirect object:
Le caffe es excellente: proba lo! - The cafe is excellent - taste it!
Dice me le conto; dice me lo (Dice le conto a me...) - Tell the story to me; tell it to me.

The reflective pronouns are used when the subject of the verb is the same as the object (direct or indirect). Like in the Romance languages, the reflective is used much more than in English:
Deo adjuta les, qui se adjuta - God helps these who help themselves
Io me sibila un melodia - I whistle a melody to myself
Tu te rasava? - Did you shave yourself?
Francese se parla in Francia - French is spoken in France

Regarding the forms of possession, the longer forms are used in constructions like le auto es le mie - the car is mine. But one can also use them more like strong adjectives than the short forms:
alicun amicos mie - some friends of mine
Matre mie! Es un piccante bolla de carne! - Mama mia! That is a spicy meatball!

Much speakers use vos instead of tu for courtesy in formal situations:
Esque vos passava un viage placente, Seniora Chan? - Did you have a nice journey, Mrs Chan?
Aperi vostre valise, Senior - Open your suitcase, sir

Use illes for groups of persons with both men and women. Illas can be used for groups of only women.

Impersonal pronouns  
Il is an impersonal pronoun for using in constructions like il pluve (it rains). Il can also be used when the real subject is a clause that occurs later in the sentence.
Il deveni tarde - It is becoming late
Il es ver que nos expende multe moneta - It is true that we spend much money
Es bon que vos veni ora - It is good that you come now

On is the pronoun used when the identity of the subject is not clear. The form for objects is uno:
On non vide tal cosas actualmente - One is not seeing such things just now
On sape nunquam lo que evenira - One knows nothing that will happen
On construe un nove linea de metro al centro urban - One constructs a new line of subway to the centre of the city
On collige le recyclabiles omne venerdi - One collects the recyclables every Friday
Tal pensatas afflige uno in le profundo del depression - Such thoughts afflict one in the depth of depression

Demonstrative pronouns 

The major forms are the adjective iste or ille and the pronoun iste, ista, and isto, that can be plural. When the subject of a sentence has two possible predecessors, iste refers to the second predecessor.
Iste vino es pessime - This wine is worst
Isto es un bon idea - This is a good idea
Janet accompaniava su soror al galeria - Janet accompanied her sister to the gallery
(a) Illa es un artista notabile - Janet is a notable artist
(b) Ista es un artista notabile - The sister is a notable artist

The adjective of distance is ille o aquelle. The pronouns are the same as the personal pronouns.
Io cognosce ille viro; ille se appella Smith - I know that man; his name is Smith
Illo es un obra magnific - That is a magnificent work

Relative and interrogative pronouns 
The relative pronouns for persons and animals are qui (subject and after prepositions) and que (object):
Nos vole un contabile qui sape contar - We want an accountant who knows counting
Nos vole un contabile super qui nos pote contar - We want an accountant on who we can count
Nos vole un contabile que le policia non perseque - We want an accountant whom the police do not pursue

The relative pronoun for things is que for all uses:

Il ha duo sortas de inventiones: illos que on discoperi e illos que discoperi uno - There are two kinds of inventions: those which you discover and those which discover you

Cuje - "whose" or "of which" - is used for persons and things:
un autor cuje libros se vende in milliones - an author whose books are sold in millions
un insula cuje mysterios resta irresolvite - an isle of which mysteries remain irresolved

All they above can be replaced by le qual or le quales:
Mi scriptorio esseva in disordine – le qual, nota ben, es su stato normal - My desk were in disorder - which, note well, is its normal state
Duo cosinos remote, del quales io sape nihil, veni visitar - Two distant cousins, of which I know nothing, come to visit

The relative pronouns are also used as interrogative pronouns.

Verbs 
These are the most common forms of verbs (note exceptions in underlined letters):

The verbs do not vary for persons and plural, except for alternative versions for esser. The tenses normal are used for the subjunctive and the command. Esse, habe, and vade have forms short for the present: es, ha, and va.

Infinitives 
Infinitives are used for both the infinitive and the gerund, and can be pluralized if needed:
Cognoscer nos es amar nos - To know us is to love us
Il es difficile determinar su strategia - It is difficult to determine his strategy
Illes time le venir del locustas - They fear the coming of the locusts
Le faceres de illa evocava un admiration general - Her actions evoked the general admiration

Simple tenses 
There are three simple tenses - past, present, and future. They are used for both the perfect (completed) and the imperfect, and for both the perfective and the non-perfective (continuing).
Io ama mangos; io mangia un justo ora - I love mangos; I am eating one right now
Mi auto es vetere e ha multe defectos: naturalmente illo va mal! - Me car is old and has many defects; naturally it goes badly!
Io vos diceva repetitemente: le hospites jam comenciava partir quando le casa se incendiava - I told you repeatedly: the guests were already beginning to leave when the house was set on fire
Nos volara de hic venerdi vespere, e sabbato postmeridie nos prendera le sol al plagia in Santorini - We will fly from here Friday evening, and Saturday afternoon we will be taking the sun at the beach in Santorini
Si ille faceva un melior reclamo, ille venderea le duple - If he made advertisings more good, he would sell twice

Participles 
Participles formation in Interlingua follows this simple pattern: 

un corvo parlante - a speaking crow
Approximante le station, io sentiva un apprehension terribile - Approaching the station, I felt an awful apprehension
un conto ben contate - a well told story
La Beltate dormiente - The Sleeping Beauty

Composed tenses  
Instead of the simple tenses, one can use composed forms. For the past, one can use ha and other forms of habe, with the past participle:
Le imperio ha cadite - The empire has fallen

For the future, use va and other forms of vade, with the infinitive:
Io va retornar - I will return

Used rarely, velle with the infinitive is the hypothetical composed:
Io velle preferer facer lo sol - I would prefer to do it alone

The passive is formed by using es or other forms of esser with the passive participle:
Iste salsicias es fabricate per experte salsicieros - These sausages are made by expert sausage makers

The passive to the Past Perfect is created by using habeva essite with the passive participle:
Nostre planeta habeva essite surveliate durante multe annos - Our planet has been surveyed for many years

Other forms 
Here are examples of various forms of commands:
Face lo ora! - Do it now!
Le imperatrice desira que ille attende su mandato - The empress wants him attend to her command
Va tu retro al campo; resta vos alteros hic - You: return to the camp; The rest of you: stay here
Cliccar hic - Click here
Que tu va via! - I want you to go away!
Que illes mangia le tortas - Let them eat cake / They can eat cakes
Que nos resta hic ancora un die / Vamos restar hic ancora un die - They must still remain here for a day / Let we still remain here for a day

Sia is the interrogative form and the subjunctive of esser:
Sia caute! - Be cautious!
Sia ille vive o sia ille morte... - Whether he is live or he is dead...
Que lor vita insimul sia felice! - Let their life together be happy!

Alternative versions of esser 
Because the verb esser is irregular in many European languages, Interlingua allows the use of versions:
es - present
sia - command and subjunctive
era - past
sera - future
serea - hypothetical

Some persons use also these forms of esser in the present tense:
son - plural
me so, nos somos (much rarely)

Verbs with two roots 
Because Interlingua is a "neo-Latin" language, it keeps any of the double roots of Latin that survive in the Romance languages and English. For example:
sentir ("to feel") > sentimento, sensor
repeller ("to repulse") > repellente, repulsive
ager ("to act") > agente, actor

Many speakers of Interlingua prefer the use of the more recognizable form in international vocabularies, and drop the other ones.

Syntax 
The normal order of words in Interlingua is subject-verb-object, but one can use other orders if the meaning is clear:
Ille reface horologios - He repairs clocks
Amandolas ama io tanto, io comprava un amandoliera - I love almonds so much that I bought an almond orchard

But pronouns tend to follow the order of the Romance languages - subject-object-verb - except for infinitives and commands, that has the object after the verb:
Ille los reface - He repairs them
Nos vole obtener lo - We want to obtain it
Jecta lo via! - Throw it away!

If two pronouns - first the object direct and second the indirect object - occur with the same verb, the indirect object goes first:
Io les lo inviava per avion - I sent it to them by plane
Io la los inviava per nave - I sent them to her by ship

Questions 
One can form questions in different ways:

1. By putting the verb before the subject:
Ha ille arrivate? - Has he arrived?
Cognosce tu ben Barcelona? - Do you know Barcelona well?
Te place le filmes de Quentin Tarantino? - Do you like the films by Quentin Tarantino?

2. By using a word of question instead of the subject:
Qui ha dicite isto? - Who has said this?
Que cadeva super te? Un incude - What fell on you? An anvil

3. By adding the particle esque (or, more rarely, an) to the beginning of the sentence:
Esque illa vermente lassava su fortuna a su catto? - Did she leave her fortune to her cat?

4. By change the tone of the sentence, or by adding a question mark, without change the normal order:
Tu jam ha finite tu labores? - Have you already finished your work?

References 
Gode, Alexander, and Hugh E. Blair. Interlingua: a grammar of the international language. Storm Publishers, New York, 1951.

Interlingua
Grammar