Ne zaboravi zakopčati kaput kad puše vjetar i otkopčati ga kad uđeš u tramvaj.

Yes. Zaboravi is the 2nd person singular imperative of zaboraviti: it means forget!
With ne, it becomes don’t forget.

So Ne zaboravi... is a natural way to tell one person informally: Don’t forget...

Croatian also often uses nemoj + infinitive for negative commands, so Nemoj zaboraviti... is also possible. In this sentence, Ne zaboravi... sounds very normal and idiomatic.

If you were speaking to:

• one person formally or more than one person: Ne zaboravite...

Because after zaboraviti Croatian commonly uses an infinitive, just like English uses to + verb in forget to do something.

So:

• zaboraviti zakopčati kaput = to forget to button up the coat
• zaboraviti otkopčati ga = to forget to unbutton it

This structure is very common:

• Ne zaboravi nazvati. = Don’t forget to call.
• Zaboravio sam ponijeti ključ. = I forgot to bring the key.

They are opposites.

• zakopčati = to fasten / button up / do up
• otkopčati = to unfasten / unbutton / undo

The core verb is related to fastening clothes. The prefixes change the meaning:

• za- often gives the idea of closing up / fastening
• ot- often gives the idea of opening / undoing

With kaput, these verbs usually mean buttoning or fastening the coat. Depending on the coat, English might also say zip up / unzip, but Croatian still often uses zakopčati / otkopčati more generally for fastening clothes.

Croatian often leaves out possessives like my, your, his when they are obvious from context.

In this sentence, it is clear that the person is being told to fasten their own coat, so kaput by itself is enough.

Also, Croatian has no articles, so there is no separate word for the or a.

So zakopčati kaput can mean:

• button up your coat
• or more literally button up the coat

Context tells you it is your coat.

Here kaput is in the accusative, because it is the direct object of zakopčati.

You are fastening what?
→ kaput

For many masculine inanimate nouns, the accusative singular looks the same as the nominative singular, so:

• nominative: kaput
• accusative: kaput

That is why the form does not visibly change.

Ga means it here. It refers back to kaput.

So:

• otkopčati ga = to unbutton it

Croatian often uses a pronoun instead of repeating the noun:

• zakopčati kaput ... i otkopčati ga instead of
• zakopčati kaput ... i otkopčati kaput

That sounds more natural and avoids repetition.

Because kaput is masculine singular, the short accusative pronoun is ga.

This is a normal Croatian word order. Literally it is when blows wind, but naturally it means when the wind is blowing or when it’s windy.

Croatian word order is much more flexible than English word order. Putting the verb before the subject is very common, especially in:

• weather expressions
• general statements
• sentences where the event is being presented first

You could also hear kad vjetar puše, but kad puše vjetar sounds very natural.

Yes. Kad puše vjetar is a general or repeated-condition statement: when the wind is blowing / when it’s windy.

Croatian often uses the present tense for things that are generally true, habitual, or expected in certain situations.

So this sentence is not only about one exact moment. It is more like practical advice:

• when it’s windy, fasten your coat
• when you get into the tram, unfasten it

Because after kad meaning when, Croatian often uses the present form to talk about a future event, especially in subordinate clauses.

So:

• kad uđeš u tramvaj = when you get into the tram

This is very similar to English, which also says:

• When you enter the tram, ... not
• When you will enter the tram...

There is also an important aspect point here:

• ući = to enter, to go in once, as a completed action
• uđeš is the present form of that perfective verb

That makes sense here because entering the tram is seen as a single completed event.

Because Croatian distinguishes between:

• movement into a place → u + accusative
• location inside a place → u + locative

Here you are entering the tram, so there is movement:

• ući u tramvaj = to get into the tram

But if you were already inside it, you would say:

• u tramvaju = in the tram

Compare:

• Ulazim u tramvaj. = I’m getting into the tram.
• Ja sam u tramvaju. = I’m in the tram.

That is just a difference in how the two languages express the idea.

English often uses:

• get on the tram
• get on the bus

Croatian usually uses:

• ući u tramvaj
• ući u autobus

Croatian treats it more literally as entering into a vehicle, especially one you can go inside. So even though the natural English translation may be get on the tram, the Croatian structure is enter into the tram.

Usually uđeš is better here.

• uđeš comes from ući and is perfective
• ulaziš comes from ulaziti and is imperfective

In this sentence, the idea is:

• first, you enter the tram
• then, once inside, you unbutton the coat

That is a single completed event, so uđeš fits well.

Kad ulaziš u tramvaj would sound more like:

• while you are getting into the tram
• or a repeated/habitual process, depending on context

So kad uđeš u tramvaj is the more natural choice here.

You would change the 2nd person singular forms to 2nd person plural/formal forms:

• Ne zaboravite zakopčati kaput kad puše vjetar i otkopčati ga kad uđete u tramvaj.

Changes:

• zaboravi → zaboravite
• uđeš → uđete

Everything else can stay the same.

A rough guide:

• š sounds like sh in shoe
  • puše
• č sounds like ch in church, but usually a bit firmer
  • zakopčati, otkopčati
• đ sounds roughly like a soft j/dj sound, somewhat like the dg in bridge or j in some pronunciations of duke
  • uđeš

A few useful word-by-word hints:

• zaboravi ≈ za-bo-RA-vi
• zakopčati ≈ za-KOP-cha-ti
• puše ≈ PU-she
• uđeš ≈ OO-jesh or OO-dyesh as a rough approximation
• tramvaj ≈ TRAM-vai

The exact sounds are not identical to English ones, but those approximations are a helpful start.