Esperanto grammar

Esperanto's grammar (rules of language) is meant to be simple. The rules in Esperanto never change and can always be applied in the same way.
Articles
Esperanto has only definite article la (the same thing as "the" in English) and no indefinite article (the same thing as "a" or "an" in English). They use definite article when they talk about things, about which they have already told something.
Nouns and adjectives
Nouns end with -o. For example, patro means father. To make a noun plural add -j. For example: patroj means fathers.
Adjectives end with -a, adverbs end with -e, for example granda means big, bona means good, bone means well.
The -n ending is the mark of the direct object (the Accusative case) in nouns and adjectives. For example:
Mi vidas vin. - I see you.
Li amas sin. - He loves her.
Ili havas belan domon. - They have a nice house.
In adjectives and adverbs is comparison made by words pli (more) and plej (most). For example:
pli granda - bigger
plej granda - biggest
pli rapide - faster
plej rapide - fastest
Pronouns
Personal pronouns are: mi - I, ci - thou singular, li - he, si - she, gi - it, ni - we, vi - you singular or plural, ili - they, oni - one/they, si (self). The pronoun oni is used for uncertain subject (like man in German). The pronoun ci means thou but people do not use it much. Instead they use vi, almost exclusively, as the singular form of you, or the plural form - you all.
Possessive pronouns are made by adding of ending -a to a personal pronoun: mia - my, cia - your singular, lia - his, sia - her, gia - its, nia - our, via - your plural, ilia - their. People use possessive pronouns like adjectives.
Accusative case (the -n ending) is used in pronouns as well: min - me, cin - thee, lin - him, sin - her, gin - it, nin - us, vin - you or you all, ilin - them. As noted with ci, cin is very seldom used in modern spoken Esperanto.
So, to say how old somebody is in Esperanto, just say:
Lia ago estas dudek = He is twenty (20) years old. (word for word: His age is twenty (20).)
Verbs
Verbs end with -as when they are in present tense. English uses I am, you are, he is. But in Esperanto, there is just one word for am, are, is - estas. Similarly, kuras can mean run or runs. Infinitives end with -i. For example, esti means to be, povi means to can. It is easy to make past tense - always add -is ending. To make future tense, add -os. For example:
kuri - to run
mi kuras - I run
vi kuras - you run
li kuris - he ran
gi kuros - it will run
Many words can be made opposite by adding mal at the beginning.
bona = good. malbona = bad
bone = well, malbone = poorly
granda = big, malgranda = small
peza = heavy, malpeza = light
Examples of sentences which show the rules:
= I can run fast.
= You cannot run fast.
= I am a boy.
= I am a bad Esperantist.
Yes/No questions
To make a yes-or-no question, add Cu at the beginning. For example:
= Do you speak Esperanto?
= Yes, I speak Esperanto very well.
= No, I am a beginner.
Unlike in English, they can answer to a yes/no question only jes (yes) or ne (no).
Numbers
The numbers are:
Numbers like twenty-one (21) are made by their compounding by order of magnitude. For example: dek tri means thirteen (13), dudek tri means twenty-three (23), sescent okdek tri means six hundred eighty-three (683), mil naucent okdek tri means (one) thousand nine hundred and eighty-three (1983).
Prefixes and suffixes
Esperanto has over 20 special words which can change the meaning of another word. People put them before or after the root of a word.
These words combined can make very long words, such as malmultekosta (cheap), vendredviandmangmalpermeso (prohibition of eating a meat on Friday).
Prefixes
Prefixes are added before the root of the word.
bo- - means "in-law". Patro means father, and bopatro means father-in-law.
dis- - means "all or many directions". Iri means to go, and disiri means to go in different directions.
ek- - means "start" of something. Kuri means to run, and ekkuri means to start running.
eks- - makes the word "former". Amiko means friend, and eksamiko means former friend.
fi- - makes the word worse. Knabo means boy, and fiknabo means bad boy; odoro means smell, and fiodoro means bad smell.
ge- - changes meaning of a word to "both gender". Frato means brother, and gefratoj means brother(s) and sister(s).
mal- - makes the word opposite. Bona means good, and malbona means bad.
mis- - means "wrong". Kompreni means to understand, and miskompreni means to understand wrong.
pra- - means "prehistoric", "very old" or "primitive". Homo means human, and prahomo means prehistoric human.
re- - means again. Vidi means to see, and revidi means to see again.
Suffixes
Suffixes are added after the root of the word, but before the ending.