Passa el drap pel mirall perquè està brut.

Here passa is the informal singular imperative of passar.

So Passa el drap pel mirall means something like:

• Run the cloth over the mirror
• Wipe the mirror with the cloth

The base verb passar often means to pass, to move across, or to run something over a surface.

In another context, passa could also be the present tense (he/she passes), but here the sentence is clearly giving an instruction, so it is understood as a command.

Catalan usually drops subject pronouns when they are not needed.

So instead of saying Tu passa..., Catalan normally just says:

• Passa el drap pel mirall

The verb form already shows that the command is addressed to you (informal singular).

Adding tu would only be for emphasis, contrast, or clarification.

El drap means the cloth or the rag.

• drap = cloth, rag
• el = the (masculine singular article)

Catalan often uses the definite article in places where English might also use the, but sometimes Catalan uses it more naturally than English does. In this sentence, el drap refers to a cloth that is understood in the situation.

Pel is a contraction of:

• per + el = pel

Here it means something like:

• over the mirror
• across the mirror
• on the mirror

So:

• Passa el drap pel mirall = Pass the cloth over the mirror

This use of per suggests movement across a surface.

Because the idea is not just toward the mirror, but across/over its surface.

Compare the basic idea:

• al mirall = to the mirror / at the mirror
• pel mirall = over/across the mirror

Since the cloth is being moved on the mirror’s surface, pel mirall is the natural choice.

Yes, it is natural and understandable.

It literally gives the image of passing the cloth over the mirror, which is a normal way to express wiping.

Other natural ways to say something similar are:

• Neteja el mirall amb un drap = Clean the mirror with a cloth
• Passa un drap pel mirall = Run a cloth over the mirror

So the sentence is perfectly normal Catalan.

Here perquè means because.

So:

• Passa el drap pel mirall perquè està brut
• Wipe the mirror with the cloth because it is dirty

In this sentence, perquè introduces the reason for the command.

This is a very common question.

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

Examples:

• Ho faig perquè vull. = I do it because I want to.
• Per què ho fas? = Why do you do it?

In your sentence, it is one word because it means because.

Està brut means is dirty.

• està = is (from estar)
• brut = dirty (masculine singular)

Catalan often uses estar for a temporary state or condition, so està brut is the normal way to say it is dirty.

Because brut is the masculine singular form of the adjective.

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

• brut = masculine singular
• bruta = feminine singular
• bruts = masculine plural
• brutes = feminine plural

In this sentence, the thing that is dirty is understood as a masculine singular noun, such as el mirall or el drap.

Grammatically, it is ambiguous.

Both nouns are masculine singular:

• el drap
• el mirall

So està brut could refer to either one.

However, in real context, the most likely meaning is usually the mirror is dirty, because that is the normal reason for wiping it. Still, the sentence itself does not completely remove the ambiguity.

If you wanted to make it clearer, you could say:

• Passa el drap pel mirall, que està brut.
• Passa el drap pel mirall perquè el mirall està brut.

Catalan, like Spanish, often distinguishes between ser and estar.

Here estar brut means to be dirty, understood as a condition or state, so estar is the normal verb.

Compare:

• És brut can suggest something more characteristic or habitual, depending on context.
• Està brut means it is dirty right now.

For a mirror that needs cleaning, està brut is the natural choice.

Yes.

The difference is:

• Passa el drap pel mirall = Pass the cloth over the mirror
  (a specific cloth, already known in context)
• Passa un drap pel mirall = Pass a cloth over the mirror
  (any cloth / a cloth)

Both are correct. The original sentence uses el drap because the cloth is treated as something specific or already understood.

Yes, it is completely normal.

The structure is:

• Passa = verb
• el drap = direct object
• pel mirall = prepositional phrase
• perquè està brut = reason clause

So the sentence is built very naturally for Catalan:

Verb + object + complement + reason

Even if English might phrase it a little differently, the Catalan word order here is standard and idiomatic.