Interlingue is a constructed language. It was called Occidental between 1922 and 1947. Edgar de Wahl, one of the first Esperantists, created it. De Wahl was from the city of Tallinn in Estonia, which was in the Russian Empire but later became its own country. He spoke German, Russian, Estonian and French since he was a child and had natural ability in languages. He is often called de Wahl.

Interlingue looks a lot like languages in the Romance language family: French, Spanish, Italian, and so on. De Wahl made a rule called De Wahl's rule that made changing verbs to nouns easier and more regular. For example, the word exploder (to explode) regularly changes its d to s (making explosion) when following this rule.

Alphabet and pronunciation 
Interlingue is written with 26 Latin letters: a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z. The letters of the alphabet are pronounced as a, be, ce, de, e, ef, ge, ha, i, jot, ka, el, em, en, o, pe, qu, er, es, te, u, ve, duplic ve, ix, ypsilon, and zet. Accents are written on the five vowels to indicate irregular stress, with the acute accent (á é í ó ú) preferred, but others (è, ê, etc.) permitted.

The vowels are usually pronounced as follows:
 a as English father.
 e open or closed, as  English bed and yes.
 i as English machine.
 o open or closed as in English door, hot.
 u as English rule, pull.
 y is a consonant after a vowel or beginning a word before a vowel, otherwise is pronounced as i.

The consonants are pronounced as in English, with the following exceptions:
 c when before e and i = [ts]: cive, helice
 g when before e and i = English s in pleasure: plage, giraffe; but elsewhere c and g are as in can, go
 ss = [s] as in pass
 s between vowels = [z]: rose, positiv
 z = [dz]
 zz = [ts]
 ch = [ʃ], English sh: chambre
 j = [ʒ], English s in pleasure
 t as [t], except when followed by ia, io, iu, or ie and not preceded by an s. Thus the t in nation is pronounced [ts] but in bastion as [t].

Other doubled consonants are pronounced as a single consonant, unless when separated they would be pronounced differently. Ex. grammatica is pronounced as if written gramatica, but occidental and suggestion are pronounced as if written as oc followed by cidental, and sug followed by gestion.

Article 
Like English, Interlingue has a definite article and an indefinite article. The definite article (the) is li, and the indefinite (a, an) is un. Plural of a noun is made by adding -s after a vowel, or -es after most consonants. To avoid pronunciation and stress changes, words ending in -c, -g, and -m only add an -s: un libre, du libres, un angul, tri angules, li tric, li trics, li plug, li plugs, li album, pluri albums, li tram, du trams.

The ending of the definite article can be modified to lo (masculine), la (feminine), lu (neuter), lis (plural), los (masculine plural), e las (feminine plural). Of these, the forms lu and lis are most common: lu in the same sense as Spanish lo and English that which, as in Ne li aprension de un lingue es lu essential, ma su usation (that which is essential is not the learning of a language, but using it), and lis to change to plural words that are difficult to make plural on their own: lis s (the s's).

Personal pronouns
Interlingue has two forms for the personal pronouns: one for the subject form (nominative), and one for the object form (accusative or dative).

Grammatical endings are used, but less than in Esperanto and Ido in which parts of speech are marked with obligatory endings. Only a few parts of speech (such as verb infinitives) in Interlingue have entirely obligatory endings, while many others either have endings the usage of which is optional and sometimes recommended. Some grammatical endings are:
 ar, er, ir: verb infinitive. far (to do), posser (be able), scrir (to write)
 e: the general noun ending used obligatorily to differentiate nouns from other parts or speech or for reasons of pronunciation. Examples of obligatory -e endings: capitale (capital, noun) vs. capital (capital, adjective), contenete (content) vs, contenet (contained), sud (south, adjective) vs. sude (south as an independent noun, as in the north and the south). A final -e is recommended in words ending with -s to avoid confusion with the plural (curse, sense), -ir, -er- and -ar endings to avoid confusion with the verb infinitive (dangere, desire, papere), and other such areas where its addition aids in differentiation or pronunciation. Optional -e endings: can or cane (dog), Pentecost or Pentecoste (Pentecost).
 i: the general adjectival ending, similar to -e in usage. Examples of obligatory -i endings: pigri (lazy) and acri (sharp) to enable pronunciation, verdi (green, adjective) to distinguish from verde (green, noun). Examples of optional -i endings: etern vs. eterni (eternal), imens vs. imensi (immense).
 a: nouns that end in e formed from an -ar verb are often written with the -a ending if one wishes to emphasize the verbal (active) aspect. A me veni un pensa (a thought occurs to me) vs. Penses e paroles (thoughts and words). The a ending also makes nouns feminine: anglese (English person), angleso (Englishman), anglesa (English woman). This does not apply to nouns that on their own indicate the gender (patre, matre).
 o: indicates the masculine gender in the same way a indicates the feminine.

Correlatives 

Notes on the correlatives:

Alcun (some) and necun (no, none) are respectively the adjectives of alquel and nequel.

The -qui series has optional accusative forms ending in -em: quem, alquem, nequem.

The -al series is adverbialized with the -men ending: qualmen (how) talmen (that way).

Correlatives can take the plural ending: queles, quales, tis, omnis, etc.

Ci (here) and ta (there) can be affixed to ti and to to indicate proximity or distance: ti libre (this book), ti-ci libre (this book here), ti-ta libre (that book there), tis (these), tis-ci (these here), tis-ta (those there), to-ci (this here), to-ta (that there).

Many derivatives are formed from the correlatives: qualitá from qual + itá, quantitá from quant + -itá, omnipotent from omni + potent.

Verbs 
Verbs in Interlingue have three endings: -ar, -er, and -ir. Conjugation is performed with a combination of endings and auxiliary verbs. The verb esser (to be) is exceptional in being written es in the present  tense, though the esse form is seen in the imperative.

Adverbs 
Interlingue has primary adverbs and derived adverbs. Primary adverbs are adverbs without special endings: tre (very), sempre (always), etc.

Derived adverbs are adverbs made by putting -men at the end of an adjective (rapid = quick, rapidmen = quickly). This is similar to -ment in French, -mente in Italian, and other Romance languages. The ending -men was inspired by Provençal and spoken French (which does not pronounce the t in -ment) and chosen over -mente to avoid clashing with the noun ending -ment and other nouns in the language derived from the past tense in -t. The ending may be omitted when the meaning is clear: tu deve far it rapid(men) = you must do it quick(ly).

Dr. F. Haas in 1956 grouped the most common adverbs by type as below.

References 

Interlingue