Vols iogurt? Sí, en vull un ara.

Vols is the 2nd person singular present form of voler (to want).

• jo vull = I want
• tu vols = you want
• ell/ella vol = he/she wants

So Vols iogurt? is addressing one person informally: Do you want yogurt?

If you were speaking to more than one person, or using the polite form, you would use voleu instead.

Catalan often drops subject pronouns when they are clear from the verb form.

Because vols already tells you the subject is tu, Catalan normally does not need to say tu.

So:

• Vols iogurt? = normal, natural
• Tu vols iogurt? = possible, but more emphatic, as in Do you want yogurt?

This is different from English, where the subject usually has to be stated.

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

• Vols iogurt? suggests some yogurt / yogurt in general
• Vols un iogurt? suggests one yogurt as a countable item, like one pot or cup of yogurt

Catalan often leaves out the article with food or substances when speaking in a more general or partitive way.

In your example, the reply Sí, en vull un ara makes it clear that the speaker means one yogurt in response.

En is a very common Catalan clitic pronoun. Here it refers back to iogurt.

With indefinite quantities or partitive meanings, Catalan often uses en where English would simply say some or would leave the noun implied.

So:

• Vols iogurt?
• Sí, en vull un.

Literally, this is something like:

• Do you want yogurt?
• Yes, of it I want one.

That literal translation sounds strange in English, but in Catalan it is completely normal.

Because they do different jobs:

• en = refers back to the noun iogurt
• un = tells you the quantity: one

So en vull un means I want one (of them / of that kind).

This is very common in Catalan:

• Tens llibres? Sí, en tinc dos. = Do you have books? Yes, I have two.
• Vols galetes? Sí, en vull una. = Do you want cookies? Yes, I want one.

English usually just says one, but Catalan often keeps both the reference pronoun and the number/article.

Because en is the pronoun normally used with indefinite or partitive reference.

Here, the idea is not I want that specific yogurt but rather I want one yogurt / one of them.

Compare:

• El vull. = I want it.
  This refers to a specific, definite thing.
• En vull un. = I want one.
  This refers to one item from a category or quantity.

So en is the correct choice in this sentence.

Because Catalan object pronouns usually go before the conjugated verb.

So:

• en vull
• el tinc
• la veig
• hi vaig

This is one of the big differences from English.

In affirmative sentences with a normal conjugated verb, clitic pronouns typically come before the verb. So en vull un is the expected order.

Grammatically, un can mean either a/an or one, depending on context.

Here it is best understood as one, because the noun iogurt is not repeated:

• en vull un = I want one
• more fully: vull un iogurt

So un is standing in for one yogurt.

Ara means now.

So Sí, en vull un ara means Yes, I want one now.

It adds a sense of time or immediacy. Without ara, the sentence would simply mean Yes, I want one.

Yes. Catalan allows some flexibility, although some orders sound more natural depending on emphasis.

For example:

• Sí, en vull un ara. = Yes, I want one now.
• Sí, ara en vull un. = Yes, now I want one. / Yes, I want one now.

The version in your sentence is perfectly natural. Moving ara changes the emphasis more than the basic meaning.

Because the speaker changes.

In the question:

• Vols iogurt? = Do you want yogurt?

In the answer:

• Sí, en vull un ara. = Yes, I want one now.

So the verb changes from:

• vols = you want to
• vull = I want

Both come from voler, but they match different subjects.

Yes, absolutely.

• Sí, en vull. = Yes, I want some / I want some of it.
• Sí, en vull un. = Yes, I want one.

So adding un makes the quantity more specific.

If the speaker wants to say one yogurt, un is useful. If they just mean some yogurt, en vull may be enough, depending on context.