Je veux que tu parles plus doucement.

Because French normally needs que to introduce a new clause after vouloir when there is a different subject.

• Je veux parler = I want to speak.
  Same subject: I want, and I speak, so French uses the infinitive.

• Je veux que tu parles = I want you to speak.
  Different subject: I want, but you speak, so French uses que + a conjugated verb.

So que is not optional here.

Because after je veux que, the verb must be conjugated, not left in the infinitive.

Here, parles is the form used with tu in a clause introduced by que after a verb of wanting. French does not say je veux que tu parler.

Compare:

• Je veux parler = I want to speak.
• Je veux que tu parles = I want you to speak.

So parler would be wrong here because tu needs a finite verb.

Yes. After vouloir que, French normally uses the subjunctive, because it expresses a wish, desire, or will.

So this is:

• Je veux que tu parles...

The tricky part is that for many -er verbs, the tu form of the present subjunctive looks exactly the same as the present indicative:

• Indicative: tu parles
• Subjunctive: que tu parles

So in this sentence, you know it is subjunctive because of the structure je veux que..., not because the form looks different.

No, French normally requires an expressed subject pronoun in ordinary sentences.

So you need:

• que tu parles

not just:

• que parles

French verbs usually do not carry enough distinct spoken endings to let you drop the subject naturally, so pronouns like je, tu, il, nous, vous are normally required.

Here, plus means more.

It is modifying the adverb doucement, so plus doucement means more softly, more quietly, or sometimes more gently / more slowly, depending on context.

This is the normal comparative pattern:

• vite = quickly

• plus vite = more quickly / faster

• doucement = softly / gently / quietly
• plus doucement = more softly / more quietly

Yes, but not here.

French plus can mean two different things:

1. more
2. no more / no longer when used in a negative structure

For example:

• Je veux que tu parles plus doucement = I want you to speak more softly.
  Here it is affirmative, so plus means more.

• Je ne veux plus que tu parles = I no longer want you to speak.
  Here ne ... plus means no longer / not anymore.

So the meaning depends on the structure.

Both are possible, but they are not exactly the same.

• doucement often means softly, gently, or quietly
• lentement means slowly

With speaking, parler doucement is very common and natural if you want someone to lower their voice or speak in a gentler way. Depending on context, it can also suggest speaking a bit more slowly and carefully.

If you specifically mean speed, plus lentement may be clearer:

• Parle plus lentement = Speak more slowly.
• Parle plus doucement = Speak more softly / quietly / gently.

Because adverbs of manner in French often come after the verb.

So:

• tu parles doucement
• tu parles plus doucement

This is very normal French word order. English often does something similar with speak softly or speak quietly.

It is an adverb formed from the adjective doux / douce meaning soft, gentle, or mild.

A common way to make French adverbs is:

• feminine adjective + -ment

So:

• douce
  • -ment → doucement

This is similar to how English often adds -ly, although the system is not identical.

It is grammatical and natural, but it can sound firm or demanding depending on tone and context.

If you want something softer or more polite, French speakers often say things like:

• Je voudrais que tu parles plus doucement.
• J’aimerais que tu parles plus doucement.
• Tu peux parler plus doucement ?
• Est-ce que tu peux parler plus doucement ?

So je veux que... is correct, but it is not always the most polite choice.

Yes. You would use vous if you are speaking to:

• one person formally, or
• more than one person

Then the sentence becomes:

• Je veux que vous parliez plus doucement.

Notice that the verb changes to parliez with vous.

So the difference is about who you are addressing, not about the basic grammar of the sentence.

A careful pronunciation would be roughly:

zhuh vuh kuh tyoo parl ploo doos-mahn

A few useful points:

• je sounds like zhuh
• veux sounds like vuh
• que is often a light kuh
• tu has the French u sound, not an English oo
• parles ends with a silent -s
• doucement ends with a nasal vowel in -ment

Also, in this sentence, the final s of plus is usually not pronounced, so it sounds like plu doucement.