Necessito un altre tovalló perquè aquest està brut.

Because Catalan often drops subject pronouns when the verb already makes the subject clear.

• necessito = I need
• The -o ending tells you it is first person singular

So Necessito... is completely natural. You could say Jo necessito..., but that usually adds emphasis or contrast, like I need another napkin, not you.

Un altre is the normal way to say another before a singular noun.

• un = a
• altre = other / another

So un altre tovalló means another napkin.

In standard Catalan, altre usually appears with some determiner:

• un altre = another
• l’altre = the other
• aquest altre = this other

That is why un altre tovalló sounds natural here.

Yes, but the nuance is slightly different.

• un altre tovalló = another napkin, often implying a replacement or a different one
• un tovalló més = one more napkin, emphasizing addition in quantity

In this sentence, un altre tovalló is especially natural because the speaker wants a new napkin because the current one is dirty.

Catalan demonstratives can be used as pronouns, not just before nouns.

So here:

• aquest = this one

Since tovalló has already been mentioned, Catalan does not need to repeat it. That makes the sentence more natural and less repetitive.

You could say perquè aquest tovalló està brut, but it sounds more repetitive than necessary.

Because aquest refers to a specific masculine singular noun nearby: tovalló.

• aquest = this one / this ... here for a masculine singular noun
• això = this in a more neutral sense, like this thing or this situation

Since the sentence is talking about this napkin, aquest is the right choice.

Because Catalan usually uses estar for a current state or condition.

• està brut = it is dirty right now
• és brut = it is dirty by nature / it is generally dirty / it’s a dirty thing

Here the speaker means the napkin is dirty at the moment, so està brut is the natural choice.

Because adjectives in Catalan usually agree in gender and number with the noun they describe.

• tovalló is masculine singular
• so the adjective must also be masculine singular: brut

Compare:

• tovalló brut = dirty napkin
• camisa bruta = dirty shirt
• tovallons bruts = dirty napkins
• camises brutes = dirty shirts

This is a very common question.

• perquè = because
• per què = why / for what reason

In this sentence, the second part gives the reason, so perquè is correct:

• Necessito un altre tovalló perquè aquest està brut. = I need another napkin because this one is dirty.

If you were asking a question, you would use per què:

• Per què necessites un altre tovalló? = Why do you need another napkin?

This is the normal and natural order.

• Necessito un altre tovalló = main statement
• perquè aquest està brut = reason

Catalan does allow some flexibility, but this version is the most straightforward. If you changed the order, it would usually be for style, emphasis, or a more marked context, not because the basic sentence requires it.

So for a learner, this is a very good standard model to follow.