Learn Hardcore French: Je veux rester calme autant que possible pendant le match. - I want to stay calm as much as possible during the game.

Rester means to stay / remain, so rester calme = to stay calm / to remain calm.
Here the idea is: the match will be stressful, and you want to keep your calm throughout it.

You could say être calme (to be calm), but rester calme is more natural when the point is not losing that state over a period of time (during the match, during an argument, etc.).

Why is there an infinitive (rester) after je veux?

In French, when two verbs share the same subject and the first one is a verb like vouloir, pouvoir, devoir, savoir, etc., the second verb stays in the infinitive:

Je veux rester calme. – I want to stay calm.
Je peux rester. – I can stay.
Je dois travailler. – I must work.

So after je veux, you normally use the infinitive (rester) if the subject is still je.

If the subject changes, you do not use an infinitive; you use que + a conjugated verb instead (see another question below).

Why is it rester calme and not something like rester la calme? Why no article before calme?

Calme here is an adjective, not a noun. It directly describes je (I):

Je veux rester calme. – I want to stay calm.

French verbs like être, rester, devenir are followed directly by an adjective when describing a state:

Je suis fatigué. – I am tired.
Elle reste silencieuse. – She stays silent.
Ils deviennent nerveux. – They become nervous.

If calme were used as a noun (meaning calmness), you would need an article, for example:

Je veux garder mon calme. – I want to keep my calm / my cool.

What does autant que possible literally mean, and are there alternatives?

Literally, autant que possible means as much as possible.

Common alternatives in this sentence would be:

Je veux rester le plus calme possible pendant le match.
Je veux rester aussi calme que possible pendant le match.

For an adjective like calme, many speakers find le plus calme possible or aussi calme que possible a bit more idiomatic than calme autant que possible, but your version is still understandable and acceptable.

Can I say aussi calme que possible instead of autant que possible?

Yes, and it is very natural:

Je veux rester aussi calme que possible pendant le match.

Structure:

aussi + adjective + que possible = as + adjective + as possible

Since calme is an adjective, aussi calme que possible fits perfectly and sounds very idiomatic.

Why doesn’t calme change if the speaker is a woman?

The adjective calme has the same spelling for masculine and feminine in the singular:

Il est calme. – He is calm.
Elle est calme. – She is calm.

So whether je refers to a man or a woman:

Je veux rester calme… is spelled the same.

Only in the plural do you add an s:

Nous voulons rester calmes. – We want to stay calm.

The s is usually silent; you only hear it in liaison (e.g. calmes et silencieux).

How would the sentence change with nous instead of je?

You would say:

Nous voulons rester calmes autant que possible pendant le match.

Changes:

je veux → nous voulons (conjugation of vouloir)
calme → calmes to agree with the plural subject nous

So the pattern is: subject–verb agreement, and adjective agreement in number.

Why is it pendant le match and not dans le match or au match?

pendant means during (indicates a time period):
- pendant le match – during the match
  This is exactly what you want here.
dans le match would mean more literally inside the match, which is not idiomatic in this sense in French.
au match means at the match (location: at that event), not necessarily during the whole duration.

Other natural options with a similar meaning to pendant le match are:

durant le match (slightly more formal)
lors du match (also more formal/literary)

Can I move pendant le match to the beginning of the sentence?

Yes:

Pendant le match, je veux rester calme autant que possible.

Time expressions like pendant le match, ce soir, demain can often go at the beginning or at the end of the sentence.
Putting it first slightly emphasizes the time frame, but the meaning is the same.

Is je veux too strong? When would I use je voudrais or j’aimerais instead?

Je veux literally means I want and expresses a firm intention or desire. In your sentence, it is about your own behavior, so it does not sound rude:

Je veux rester calme… – I want to stay calm…

It can sound too direct if you are asking for something from someone else:

Je veux un café. can sound bossy.

More polite or softer options in that kind of context:

Je voudrais un café. – I would like a coffee.
J’aimerais un café. – I would like / I’d love a coffee.

In your example, all three are possible with slightly different nuances of determination vs wish:

Je veux rester calme… – strong intention
Je voudrais rester calme… – I would like to stay calm (a bit softer)
J’aimerais rester calme… – I wish I could stay calm

What tense is veux, and how is vouloir conjugated?

Veux is the present tense (présent de l’indicatif) of vouloir for je (and also tu).

Present of vouloir:

je veux – I want
tu veux – you want (singular, informal)
il / elle / on veut – he / she / one wants
nous voulons – we want
vous voulez – you want (plural or formal)
ils / elles veulent – they want

In this sentence, the present tense expresses a current intention about a future situation (the match), similar to English.

How do I say I want you to stay calm instead of I want to stay calm?

You need que + a conjugated verb, because the subject changes:

Je veux que tu restes calme pendant le match. – I want you to stay calm during the match.

Pattern:

Je veux que + [other subject] + [verb in subjunctive].

Here restes is in the subjunctive, but for tu, the form restes looks the same as the normal present, so you do not see a difference in writing.

More polite versions:

Je voudrais que tu restes calme pendant le match.
J’aimerais que tu restes calme pendant le match.

Can I say Je veux rester calme autant que je peux? Is that wrong?

It is not wrong, but it is less standard than autant que possible or le plus possible.

Je veux rester calme autant que je peux pendant le match.
= I want to stay calm as much as I can during the match.

Nuance:

autant que possible / le plus possible → as much as is possible in general, maximum in an absolute sense.
autant que je peux → as much as I am able to, focusing more on my own ability / limits.

French speakers more commonly use:

Je veux rester le plus calme possible pendant le match.
Je veux rester aussi calme que possible pendant le match.
Je veux rester calme autant que possible pendant le match.