Vull que em preguntis si no entens la resposta.

Because after vull que (I want that / I want ... to), Catalan normally uses the subjunctive if the subject of the second verb is different.

• vull = I want
• que = introduces the subordinate clause
• preguntis = you ask (present subjunctive, tu form)

So:

• Vull que preguntes... would sound wrong here.
• Vull que preguntis... is the expected form.

This is one of the most important patterns in Catalan:

• Vull que vinguis = I want you to come
• Espero que ho entenguis = I hope you understand it

Em means me.

In em preguntis, it shows who the question is directed to:

• preguntar = to ask
• preguntar-me / em preguntar = to ask me

So que em preguntis means that you ask me.

Catalan object pronouns usually go before the conjugated verb:

• em preguntis = you ask me
• et truco = I call you
• li dono = I give him/her

Because in normal finite clauses, Catalan weak pronouns usually come before the verb.

So Catalan says:

• em preguntis
• literally: me ask

where English says:

• ask me

This is very normal in Catalan and other Romance languages. Compare:

• Em veu = He sees me
• Et conec = I know you
• Li escric = I write to him/her

With infinitives, the pronoun can attach differently:

• preguntar-me = to ask me

But in your sentence, the verb is conjugated, so em comes before preguntis.

Because si no and sinó are different things.

si no = if ... not

This is what you have here:

• si no entens la resposta = if you do not understand the answer

sinó = but rather / except

Examples:

• No és blau, sinó verd = It isn’t blue, but rather green
• No vull cafè sinó te = I don’t want coffee, but tea

So in your sentence, it must be si no because it introduces a condition: if you don't understand.

Because after si meaning if, Catalan normally uses the indicative for real or possible conditions in the present.

So:

• si no entens = if you don’t understand

This is the normal pattern.

The subjunctive appears after vull que, but not after this si clause.

So the sentence contains both moods:

• Vull que em preguntis → subjunctive, because it depends on want
• si no entens la resposta → indicative, because it is a normal if clause

Entens is the 2nd person singular present indicative of entendre (to understand).

It corresponds to tu:

• jo entenc = I understand
• tu entens = you understand
• ell/ella entén = he/she understands

So si no entens means if you do not understand.

A learner may notice that the endings are not all regular-looking, and that is true: entendre has some stem changes, so it is worth learning as a common verb.

Because Catalan usually uses que to introduce a subordinate clause after verbs like want, hope, prefer, etc.

• Vull que em preguntis...
• literally: I want that you ask me...

English often uses an infinitive:

• I want you to ask me...

But Catalan normally prefers:

• vull que + subjunctive

So que is required here.

Catalan uses the definite article more often than English, especially when referring to something specific and known from context.

Here, la resposta means the answer, referring to a particular answer already being discussed.

So:

• la resposta = the answer

Leaving out the article would usually sound unnatural here.

This is very common in Catalan:

• M’agrada la música = I like music
• No entenc la pregunta = I don’t understand the question

English often omits the article where Catalan keeps it.

It means ask me, and from context that usually implies ask me a question or ask me about it.

The verb preguntar can be used with:

• a person: preguntar a algú / preguntar algú depending on structure and style
• a thing: preguntar una cosa

In this sentence, em preguntis focuses on the person being asked: ask me.

The exact thing being asked is left implicit, but the context makes it clear:

• ask me if you do not understand the answer
• in other words, ask me for clarification

No, it is not stated with tu, but it is understood from the verb forms.

Catalan often drops subject pronouns because the verb ending already shows the person:

• preguntis = you ask
• entens = you understand

So Catalan does not need to say tu unless it wants emphasis or contrast.

A more emphatic version could be:

• Vull que tu em preguntis si no entens la resposta

But normally tu is omitted.

Not in this sentence.

After si meaning if, the normal form here is the indicative:

• si no entens la resposta

Using entenguis would sound wrong in standard Catalan in this context.

So the contrast is:

• vull que em preguntis → subjunctive, because it depends on want
• si no entens la resposta → indicative, because it is an ordinary conditional clause

This is a very useful distinction to learn.

It breaks down like this:

• Vull = I want
• que em preguntis = that you ask me
• si no entens la resposta = if you do not understand the answer

So the structure is:

main clause

• Vull

• subordinate clause with subjunctive

• que em preguntis

• conditional clause
• si no entens la resposta

This makes it a good example of how Catalan combines:

1. a verb of wanting,
2. a subjunctive clause,
3. an if clause.

Yes, it is natural and clear.

It sounds like something a teacher, tutor, or helpful speaker might say:

• Vull que em preguntis si no entens la resposta.

It is a bit more personal and direct than some alternatives. Depending on tone, a speaker might also say:

• Si no entens la resposta, pregunta-m’ho / pregunta’m.
• Pregunta’m si no entens la resposta.

But your original sentence is perfectly good Catalan and especially natural when emphasizing I want you to ask me.