No parlis tan fort perquè la meva filla pugui dormir.

Because this is a negative command: No parlis = Don’t speak.

In Catalan, negative commands are formed with the present subjunctive, not the ordinary present indicative. So:

• parles = you speak / you are speaking
• no parlis = don’t speak

Compare:

• Parla més baix. = Speak more quietly.
• No parlis tan fort. = Don’t speak so loudly.

So parlis is not describing an action; it is part of the command form.

Tan fort means so loudly or that loudly.

• tan = so / that
• fort literally means strong, but with speaking or sound it often means loud

So:

• parlar fort = to speak loudly
• tan fort = so loudly

A native English speaker may expect something like fortament, but Catalan usually uses fort naturally in this context.

This part means so that my daughter can sleep.

Here, perquè introduces a purpose clause: the reason for the request. The idea is:

• Don’t speak so loudly, so that my daughter can sleep.

After perquè when it expresses purpose, Catalan normally uses the subjunctive:

• pugui = subjunctive of poder

So:

• perquè pugui dormir = so that she can sleep

No. Perquè can mean different things depending on context.

In this sentence, it means so that, expressing purpose.

Two very common uses are:

• perquè + indicative/subjunctive depending on meaning = because / so that
• context tells you which one it is

Examples:

• Ho faig perquè vull. = I do it because I want to.
• Calla perquè el nen pugui dormir. = Be quiet so that the child can sleep.

So in your sentence, it is not giving the cause of the loud speaking; it is giving the goal of the request.

Because after perquè with a purpose meaning, Catalan uses the subjunctive, not the indicative.

• pot = indicative: can
• pugui = subjunctive form of poder

So:

• la meva filla pot dormir = my daughter can sleep
• perquè la meva filla pugui dormir = so that my daughter can sleep

The subjunctive is used because this is not just a plain statement of fact; it expresses something intended, desired, or aimed at.

Catalan often uses the definite article before a possessive:

• la meva filla = my daughter
• el meu pare = my father
• la nostra casa = our house

This is more normal in Catalan than in English. English says my daughter, but Catalan usually says the my daughter, literally speaking.

So la meva filla is the standard natural form.

Because it follows poder.

In Catalan, just like in English, modal verbs such as poder are followed by an infinitive:

• pot dormir = can sleep
• pugui dormir = may/can sleep

So:

• pugui is the conjugated verb
• dormir stays in the infinitive

That is why you do not get pugui dormi.

Not for the standard meaning Don’t speak so loudly.

No parles looks like the present indicative form (you do not speak) or may sound nonstandard/dialectal in this use, but in standard Catalan the normal negative command is:

• No parlis

So if you want to tell someone not to do something, use:

• No + present subjunctive

Examples:

• No corris. = Don’t run.
• No cridis. = Don’t shout.
• No parlis tan fort. = Don’t speak so loudly.

Yes. Catalan often leaves out subject pronouns when the verb already shows the person.

So:

• No parlis already means you (singular) don’t speak
• there is no need to say tu

You could say Tu no parlis tan fort for emphasis, but normally it is omitted.

Yes. Fort has several related meanings depending on context:

• un home fort = a strong man
• una olor forta = a strong smell
• parlar fort = to speak loudly
• música forta = loud music

So in this sentence, the context of speaking makes fort mean loud, not strong.

Yes. A more formal alternative for the purpose clause would be:

• No parlis tan fort perquè la meva filla pugui dormir.
• No parlis tan fort per tal que la meva filla pugui dormir.

Per tal que is more explicitly so that and sounds a bit more formal or careful.

But the original sentence is completely natural and correct.

Then the command form would change to the vosaltres form:

• No parleu tan fort perquè la meva filla pugui dormir.

Here:

• parlis = don’t speak (to one person)
• parleu = don’t speak (to more than one person)

The rest of the sentence stays the same.

It breaks down like this:

• No parlis = Don’t speak
• tan fort = so loudly
• perquè = so that
• la meva filla = my daughter
• pugui dormir = can sleep / may sleep

So the structure is:

negative command + degree expression + purpose clause

In other words:

Don’t speak so loudly so that my daughter can sleep.

In more natural English, we would usually say:

Don’t speak so loudly so that my daughter can sleep or more naturally Don’t speak so loudly so my daughter can sleep.