Stavi čestitku u kuvertu, molim te.

Stavi is the imperative form, used for a command or request to one person.

It comes from staviti = to put / to place.

So:

• stavi = put!
• staviš = you put
• stavlja = he/she puts

Croatian also cares about aspect. Staviti is perfective, so stavi suggests do this as one complete action: put the card into the envelope. That is exactly what this sentence needs.

It is informal.

There are two clues:

• stavi is the imperative for one person
• molim te uses te, which means you when speaking informally to one person

A more formal or plural version would be:

• Stavite čestitku u kuvertu, molim vas.

That can mean:

• speaking politely to one person, or
• speaking to more than one person

Because both nouns are in the accusative singular.

Their basic forms are:

• čestitka = card
• kuverta = envelope

These are feminine nouns ending in -a, and in the accusative singular they usually change:

• -a → -u

So:

• čestitka → čestitku
• kuverta → kuvertu

But they are accusative for different reasons:

• čestitku is the direct object: the thing you are putting
• kuvertu is accusative because u here means movement into something

Because Croatian uses different cases after u depending on the meaning.

• u + accusative = movement into
• u + locative = location in / inside

So:

• Stavi čestitku u kuvertu. = Put the card into the envelope.
• Čestitka je u kuverti. = The card is in the envelope.

That is a very important Croatian pattern.

It means please in this sentence, but literally it is closer to I ask you or I beg you.

Breakdown:

• molim = I ask / I beg
• te = you (informal singular, accusative)

As a fixed everyday phrase, molim te is just a natural way to say please to one person.

The formal version is:

• molim vas

Te is the accusative form of ti = you (informal singular).

So:

• ti = you
• te = you (object form)

In molim te, the structure is basically:

• molim = I ask
• te = you

So literally: I ask you.

Because molim te is being used as a polite parenthetical expression, like please in English.

So Croatian usually separates it with a comma:

• Stavi čestitku u kuvertu, molim te.

If it comes at the beginning, the comma usually comes after it:

• Molim te, stavi čestitku u kuvertu.

Both are natural.

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

The given sentence is a very normal, neutral order:

• Stavi čestitku u kuvertu, molim te.

You can also say:

• Molim te, stavi čestitku u kuvertu.

That sounds just as natural.

Other word orders are possible too, but they can add emphasis or sound less neutral. For learners, the safest choices are:

• Stavi čestitku u kuvertu, molim te.
• Molim te, stavi čestitku u kuvertu.

Because Croatian does not have articles.

English says:

• put the card in the envelope
• put a card in an envelope

Croatian usually just says:

• stavi čestitku u kuvertu

The exact meaning depends on context. Croatian speakers understand from the situation whether you mean a or the.

In this sentence, čestitka means greeting card or simply card.

Be careful, though: the word is related to congratulating, so in other contexts it can also refer to congratulations.

Here, because it is something you put in an envelope, the meaning is clearly card.

No. Kuverta is very common in everyday speech, but you may also see:

• omotnica = envelope

So both can be used, though omotnica may sound a bit more standard or formal in some contexts.

For example:

• Stavi čestitku u kuvertu.
• Stavi čestitku u omotnicu.

Both are understandable.

You could, but it would change the feel.

• staviti / stavi = perfective, one completed action
• stavljati / stavljaj = imperfective, repeated or ongoing action

Here, you are asking for one simple completed action, so stavi is the natural choice.

Stavljaj čestitku u kuvertu would sound odd in this context, because it suggests something repeated, ongoing, or habitual, which is not what you normally mean here.