Porta una goma i un llapis a classe, perquè jo avui no en tinc cap.

Here porta is the informal singular imperative of portar, meaning bring.

So the sentence is speaking to one person in a familiar way:

• Porta... = Bring...

Catalan often leaves out the subject pronoun, so tu porta... would normally just be porta...

A useful note: porta can also be a present-tense form meaning he/she brings or you bring in some contexts, but here the context clearly makes it a command.

Because Catalan nouns have grammatical gender, and the indefinite article must match:

• una goma = an eraser
• un llapis = a pencil

Here:

• goma is feminine
• llapis is masculine

So Catalan uses:

• un for masculine singular nouns
• una for feminine singular nouns

Yes. In Catalan, goma commonly means eraser or rubber.

That can be confusing for English speakers because gum in English usually means something else. In Catalan:

• goma = eraser / rubber
• chewing gum is usually xiclet

So in a school-supplies context, goma definitely means eraser.

In this sentence, a classe is a very natural fixed expression meaning to class or to school/classroom contextually.

Catalan often omits the article in expressions like this, especially when referring to the activity or destination in a general way:

• anar a classe = to go to class
• ser a classe = to be in class
• porta ... a classe = bring ... to class

If you say a la classe, it tends to sound more like a specific classroom or a particular class as a physical place. Here the article-less form is the natural one.

Here perquè means because.

So:

• ..., perquè jo avui no en tinc cap. = ..., because I don’t have any today.

This is different from per què as two words, which usually means why or for which depending on context:

• Per què arribes tard? = Why are you arriving late?

A useful shortcut:

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

Yes, Catalan often drops subject pronouns because the verb ending already shows who the subject is.

So tinc already means I have, and the sentence could have said:

• ..., perquè avui no en tinc cap.

That would still be correct.

Adding jo gives a bit more emphasis or contrast, something like:

• ..., because I don’t have any today
• or even ..., because I myself don’t have any today

So jo is not required, but it sounds natural if the speaker wants a little emphasis.

Avui means today.

It is an adverb of time, and it tells you when the speaker does not have the items:

• jo avui no en tinc cap = I don’t have any today

Catalan adverbs like avui can often move around somewhat without changing the basic meaning:

• Jo avui no en tinc cap
• Avui jo no en tinc cap
• Jo no en tinc cap avui

The version in your sentence sounds very natural.

En is a very important Catalan pronoun. Here it replaces the previously mentioned things: una goma i un llapis.

So instead of repeating the nouns, Catalan uses en:

• Porta una goma i un llapis..., perquè jo avui no en tinc cap.

Literally, it is something like:

• Bring an eraser and a pencil..., because today I don’t have any of them.

This en is often called a partitive pronoun. It is commonly used when talking about:

• some amount of something
• some items already mentioned
• none / any of them

A key pattern is:

• tenir-ne = to have some / any of it or them
• No en tinc cap = I don’t have any

Because Catalan object pronouns like en normally go before the conjugated verb:

• en tinc
• en vols
• en comprarem

Meanwhile, cap is not a clitic pronoun. Here it works like a negative/indefinite determiner meaning any or none, so it appears in its usual position after the verb:

• no en tinc cap

So the structure is:

• no
  • en
    • tinc
      • cap

This word order is completely normal in Catalan.

Here cap means any or none, depending on how you translate the whole negative sentence.

So:

• no en tinc cap = I don’t have any
• more literally: I have none of them

In positive-looking contexts like questions or conditionals, cap can also mean any:

• Tens cap problema? = Do you have any problem?
• Si tens cap dubte... = If you have any doubt...

But in your sentence, because the clause is negative, the natural English meaning is none / not any.

Yes, from an English-speaker’s point of view it can look like a double negative, but in Catalan this is normal. Catalan uses negative concord, which means negative words often appear together in the same clause.

So:

• no ... cap
• no ... mai
• no ... ningú
• no ... res

are all normal patterns.

In your sentence:

• no en tinc cap

this is the standard way to say I don’t have any.

English usually uses just one negative marker, but Catalan commonly combines them. So it is better not to think of it as a mistake or as an emphatic double negative; it is simply how standard Catalan grammar works.

Yes, that would also be possible, but it is less compact and repeats the nouns.

Your sentence is more natural because once una goma i un llapis have already been mentioned, Catalan prefers to avoid repetition by using en:

• no en tinc cap

A fuller version might be something like:

• ..., perquè avui no tinc cap goma ni cap llapis.

That is grammatical, but the original sentence sounds smoother and more idiomatic because it uses the pronoun to refer back to the items already mentioned.