No posis res a la bústia si falta el segell.

Because this is a negative command addressed to tu.

In Catalan, negative commands use the present subjunctive, not the ordinary present indicative:

• poses = you put / you place
• no posis = don’t put

So:

• Posa-ho = Put it
• No ho posis = Don’t put it

That is why the sentence begins with No posis...

Res literally often means nothing, but in sentences like this it is often best understood as anything.

So:

• No posis res... = Don’t put anything...

This is very common in Catalan. After a negative, words like res can correspond to English anything or nothing, depending on how English naturally expresses it.

Examples:

• No veig res = I don’t see anything
• No tinc res = I don’t have anything

Because a is commonly used to indicate the destination of putting something somewhere.

With verbs like posar (to put), Catalan often uses a:

• posar una carta a la bústia = to put a letter in the mailbox

English uses in, but Catalan often uses a where English would use in or into.

So a la bústia is the normal way to say in/into the mailbox here.

Bústia usually means mailbox, letter box, or postbox, depending on the variety of English you prefer.

In this sentence, it most naturally means the place where you deposit mail.

So a la bústia means something like:

• into the mailbox
• in the letter box
• into the postbox

Literally, falta el segell means the stamp is missing or the stamp is lacking.

The important point is that el segell is the subject of the verb faltar.

So Catalan structures it as:

• falta el segell = the stamp is missing

not as:

• someone lacks the stamp

This is a very common pattern with faltar. The thing that is absent is the grammatical subject.

Examples:

• Falta sal = There’s no salt / Salt is missing
• Falten diners = Money is missing

Because the sentence refers to the stamp that the item is supposed to have.

Using el segell sounds natural because it means the required, relevant stamp for that letter or item of mail. English often does the same:

• if the stamp is missing

rather than:

• if a stamp is missing

So el segell does not necessarily mean one specific stamp already mentioned earlier; it can also mean the expected stamp in this situation.

Because after si meaning if, Catalan normally uses the indicative for real or possible conditions of this kind.

So:

• si falta el segell = if the stamp is missing

This is normal. Catalan does not usually use the subjunctive after si in ordinary conditional clauses like this.

Compare:

• Si plou, no sortim = If it rains, we don’t go out
• Si falta el segell, no posis res a la bústia = If the stamp is missing, don’t put anything in the mailbox

Yes, segell can mean seal in some contexts, but here it clearly means a postal stamp.

The context tells you that:

• bústia = mailbox/postbox
• putting something in the mailbox requires postage
• so el segell is the stamp

Catalan words often cover more than one meaning, just like English words do.

Yes, that is possible if the location has already been mentioned or is understood from context.

For example:

• A la bústia, no hi posis res si falta el segell.

Here hi refers back to a la bústia.

But in your original sentence, Catalan simply states the place directly:

• No posis res a la bústia...

That is perfectly natural and clear.

Because Catalan usually leaves subject pronouns out when they are already clear from the verb ending.

Here, posis tells you the sentence is addressing tu.

So:

• No posis... = Don’t you put... / Don’t put...

Catalan normally omits tu unless it is needed for emphasis, contrast, or clarity.

Posar is the standard, neutral verb for to put/place and is a very good choice here.

Ficar can also mean to put, and in some regions it is very common in everyday speech. But posar is more broadly standard and safer for learners.

So:

• No posis res a la bústia... = very standard
• No fiquis res a la bústia... = possible in colloquial speech in some varieties

For learning general Catalan, posar is the best verb to focus on first.