Ta marka više ne vrijedi, pa sam na kuvertu morala staviti novu.

Here marka means stamp, specifically a postage stamp.

Croatian marka can mean several things depending on context, including:

• brand / make
• stamp
• historically, a mark as a currency name

Because the sentence mentions an envelope and putting a new one on it, the meaning is clearly postage stamp. If you wanted to be completely explicit, you could say poštanska marka.

Ta means that.

It is a demonstrative word that must agree with the noun:

• ta = feminine singular
• marka = feminine singular

So ta marka means that stamp.

You can compare:

• taj čovjek = that man
• ta kuverta = that envelope
• to pismo = that letter

Više ne means no longer or not anymore.

So:

• Ta marka više ne vrijedi = That stamp is no longer valid

Literally, više often means more, but in the pattern više ne + verb, it means no longer.

Examples:

• Više ne živim tamo. = I no longer live there.
• Više ne radim ovdje. = I don't work here anymore.

Vrijedi is the 3rd person singular present form of vrijediti.

This verb can mean different things depending on context:

• to be worth
• to be valid
• to count / apply

In this sentence, it means to be valid:

• Ta marka više ne vrijedi. = That stamp is no longer valid.

So the idea is that the stamp can no longer be used for postage.

Here pa means so, therefore, or and so.

It connects the two ideas:

• the stamp is no longer valid
• as a result, the speaker had to put on a new one

So:

• Ta marka više ne vrijedi, pa...
  = That stamp is no longer valid, so...

In other contexts, pa can also mean something closer to then, well, or just function as a connector in conversation.

Sam morala is part of the past tense.

Croatian past tense is usually made with:

• the auxiliary biti in the present tense
• plus the past participle

Here:

• sam = I have / am as the auxiliary for I
• morala = past participle of morati in the feminine singular

Together, sam morala means I had to.

Then it is followed by the infinitive:

• staviti = to put

So:

• sam morala staviti = I had to put

Because the speaker is female.

In Croatian past tense, the participle agrees with the gender of the subject:

• morao sam = I had to (male speaker)
• morala sam = I had to (female speaker)

In this sentence, the word order is:

• sam ... morala

but it still means the speaker is female.

Because sam is a clitic, and Croatian clitics usually appear very early in the clause, often in the second position.

So instead of saying:

• pa morala sam...

Croatian normally prefers:

• pa sam ... morala...

This is a normal Croatian word-order pattern. English learners often notice it because it feels different from English.

Because na can take different cases depending on the meaning.

• na + accusative = movement onto a surface
• na + locative = location on a surface

Here the stamp is being put onto the envelope, so Croatian uses the accusative:

• na kuvertu = onto the envelope

Compare:

• Stavila sam marku na kuvertu. = I put the stamp onto the envelope.
• Marka je na kuverti. = The stamp is on the envelope.

So:

• kuvertu = accusative
• kuverti = locative

Novu means a new one, and the noun is omitted because it is obvious from context.

The full version would be:

• staviti novu marku = put a new stamp

But Croatian often leaves out a repeated noun when it is clear, just like English can say a new one.

So:

• staviti novu
  literally = to put a new one
• understood meaning = to put a new stamp

Also, novu is feminine singular accusative, agreeing with the omitted noun marku.

Yes. Croatian word order is much more flexible than English word order.

This sentence puts na kuvertu before morala staviti novu, which is perfectly natural. It may help highlight the place where the new stamp had to be put.

A more neutral-feeling alternative could be:

• Ta marka više ne vrijedi, pa sam morala staviti novu na kuvertu.

Both are grammatical. The difference is mainly one of focus and style, not basic meaning.

Croatian often moves elements around more freely than English, especially adverbials like na kuvertu.

Yes, kuverta is a normal and widely understood word for envelope.

You may also see:

• omotnica

That is another Croatian word for envelope, often felt as more formal or more standard in some contexts.

So both can occur, but kuverta is very common in everyday speech.

Not very naturally in English.

Because vrijedi can mean counts, is worth something, or is valid, a literal translation might sound odd. In this context, the most natural English meaning is:

• That stamp is no longer valid, so I had to put a new one on the envelope.

So even though the Croatian verb is broad in meaning, the correct English translation here is about validity for use, not about counting.