Mi volas venki mian kontraŭulon.

Because volas is the finite verb, and it has to agree with the subject in tense.

• mi = I
• volas = want / am wanting

In Esperanto, the present tense always ends in -as, no matter who the subject is:

• mi volas = I want
• vi volas = you want
• li/ŝi volas = he/she wants

The dictionary form is voli = to want, but in a sentence you normally use the conjugated form volas.

Because after volas, Esperanto usually uses an infinitive.

So:

• volas venki = want to win / want to defeat

Here, volas is the main conjugated verb, and venki is the infinitive to defeat / to win against.

This works like English:

• I want to eat → Mi volas manĝi
• I want to go → Mi volas iri
• I want to defeat → Mi volas venki

If you said Mi volas venkas, that would be ungrammatical.

Because it describes a direct object, and in Esperanto direct objects take -n.

The direct object here is kontraŭulon = opponent. Since mian modifies kontraŭulon, it must also match it and take -n:

• mia kontraŭulo = my opponent
• mian kontraŭulon = my opponent (as the direct object)

This matching is very important in Esperanto. Adjectives and possessive-like words such as mia, via, lia, etc. agree with the noun in both number and accusative marking.

Because it is the direct object of venki.

In Mi volas venki mian kontraŭulon, the person being defeated is the object of the action. Esperanto marks that with -n:

• kontraŭulo = opponent
• kontraŭulon = opponent as a direct object

So the structure is:

• Mi = subject
• volas = main verb
• venki = infinitive
• mian kontraŭulon = direct object of venki

It is built from parts:

• kontraŭ = against
• -ul- = person characterized by something
• -o = noun ending

So kontraŭulo literally means something like a person against you/another, which naturally becomes opponent, adversary, or rival.

This kind of word-building is very common in Esperanto and helps learners guess meanings.

In this sentence, it is best understood as to defeat.

Venki iun means to defeat someone. Since the object is a person (mian kontraŭulon), the meaning is clearly defeat my opponent.

Compare:

• Mi volas venki mian kontraŭulon = I want to defeat my opponent
• Mi volas gajni la ludon = I want to win the game

A useful distinction is:

• venki iun = defeat someone
• gajni ion = win something

In real usage, venki can sometimes also correspond to win, but with a person as the object, defeat is the clearest translation.

Yes, to some extent. Esperanto word order is fairly flexible because the -n ending shows the object.

The neutral order is:

• Mi volas venki mian kontraŭulon.

But you could also say:

• Mian kontraŭulon mi volas venki.

That puts more emphasis on my opponent.

Even so, the most natural order for a beginner is the straightforward one: subject + verb + infinitive + object.

Because Esperanto verbs do not change according to person or number.

In English we say:

• I want
• he wants

But in Esperanto:

• mi volas
• li volas
• ili volas

The ending -as always marks the present tense, regardless of the subject. This is one of the simplest parts of Esperanto grammar.

A simple approximate pronunciation is:

• Mi → mee
• volas → VOH-lahs
• venki → VEN-kee
• mian → MEE-ahn
• kontraŭulon → roughly kon-TRAW-oo-lon

About aŭ:

• It is a diphthong, pronounced roughly like ow in cow

So kontraŭ sounds roughly like KON-traw.

Also, Esperanto stress normally falls on the second-to-last syllable:

• kon-tra-ŬU-lon? Actually syllabification matters here: kon-traŭ-u-lon
• The stress is on u? No, the stress is on the next-to-last syllable, so kon-traŭ-U-lon

So kontraŭulon is stressed on u: kon-traŭ-U-lon.

Because Esperanto infinitives do not need a separate word like English to.

In English:

• to win
• to defeat

In Esperanto, the infinitive is shown by the ending -i:

• venki = to defeat / to win

So volas venki already means want to defeat. There is no extra word needed.

It behaves very much like one.

Words like mia, via, lia, ŝia, nia, ilia are often called possessive pronouns in English grammar, but in Esperanto they function adjectivally when placed before a noun:

• mia kontraŭulo = my opponent
• miaj kontraŭuloj = my opponents
• mian kontraŭulon = my opponent (object)

Notice how mia changes to match the noun, just like an adjective would:

• singular/plural
• with or without -n

So for practical learning, it is very helpful to think of mia as an adjective-like word.