Demà tornaré a correus perquè encara em falta un segell.

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

• tornaré = I will return / I will go back
• The ending -é tells you it is first person singular.

So jo is not necessary here. You could say Jo tornaré... for emphasis, but the neutral version usually leaves it out.

Tornaré is the future tense of tornar.

In this sentence, tornar means to return / to go back, not to turn.

So:

• torno = I return / I go back
• tornaré = I will return / I will go back

Here it means the speaker will go back to the post office another time.

Because the action is going to happen tomorrow, Catalan naturally uses the future:

• Demà tornaré = Tomorrow I’ll go back

Catalan can sometimes use the present for future meaning, especially in informal speech, but the future tense is very normal and clear here.

For example:

• Demà torno a correus = Tomorrow I’m going back to the post office
• Demà tornaré a correus = Tomorrow I’ll go back to the post office

Both can work, but tornaré is especially straightforward.

A correus is an idiomatic way to say to the post office.

A few useful points:

• correus literally relates to mail/post
• In everyday Catalan, anar a correus is a set expression meaning to go to the post office
• You may also hear fuller expressions like anar a l’oficina de correus

So a correus is not something to translate word-for-word too mechanically. It works as a standard destination phrase.

You may also sometimes see Correus with a capital C when referring to the postal service as an institution or brand.

Because perquè means because.

This is a very common Catalan distinction:

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

So in this sentence:

• ...perquè encara em falta un segell = ...because I still need a stamp

If it were a question, you might get per què instead:

• Per què tornes a correus? = Why are you going back to the post office?

Here encara means still.

So:

• encara em falta un segell = I still need a stamp

It gives the idea that this is an unresolved situation: the speaker needed a stamp before, and that is still true now.

Depending on context, encara can also mean things like yet or even, but still is the right sense here.

The verb faltar often works differently from English.

Literally, em falta un segell is closer to:

• a stamp is missing to me

But in natural English, we usually say:

• I still need a stamp
• I’m still missing a stamp

Structure:

• em = to me
• falta = is lacking / is missing
• un segell = a stamp

So the thing that is missing, un segell, is the grammatical subject of falta.

Compare:

• Em falta un euro = I’m missing one euro / I still need one euro
• Em falten dos segells = I’m missing two stamps

Notice how the verb changes:

• falta with singular
• falten with plural

Because em is an unstressed object pronoun, and in Catalan these pronouns usually go before a conjugated verb.

So:

• em falta = I need / is missing to me
• et falta = you need / is missing to you
• li falta = he/she needs / is missing to him/her

This is normal Catalan clitic placement.

With an infinitive, pronouns can attach to the end:

• faltar-me = to be missing to me

But with a normal conjugated form like falta, the pronoun comes before:

• em falta

Here segell means stamp, specifically a postage stamp.

So:

• un segell = a stamp

The indefinite article un works just like English a here.

Also note that segell can mean seal in other contexts, but in a sentence about going to the post office, stamp is the natural meaning.

Catalan word order is fairly flexible, though this sentence is very natural as it stands.

Original:

• Demà tornaré a correus perquè encara em falta un segell.

You could also say:

• Tornaré a correus demà perquè encara em falta un segell.

That said, putting Demà first is very common because it sets the time immediately.

Inside the second clause, encara em falta un segell is also the most natural order. You might move things around for emphasis, but learners should treat the original order as the safe, standard one.

Yes. Both are possible, but they are slightly different in feel.

• em falta un segell = I’m still missing a stamp / I still need a stamp
• necessito un segell = I need a stamp

Em falta emphasizes that something is lacking to complete what you need. In this sentence, it fits very well because the speaker has a reason for going back: one item is still missing.

So encara em falta un segell sounds especially natural here.