Nakon prve rečenice stavi zarez, a na kraju točku.

Nakon means after. It is a preposition that requires the genitive case in Croatian.

So the basic form rečenica becomes rečenice after nakon:

• rečenica = sentence
• nakon rečenice = after the sentence

Because prva has to agree with rečenica, it also changes:

• prva rečenica = the first sentence
• nakon prve rečenice = after the first sentence

Because the phrase is in the genitive singular, not the nominative.

The dictionary/basic form is:

• prva rečenica = the first sentence

But after nakon, both words must change to genitive singular:

• prve rečenice

This is just adjective-noun agreement:

• prva → prve
• rečenica → rečenice

Both are feminine singular here.

Stavi is the imperative form, meaning put or place.

It comes from the verb staviti = to put, to place.

Here it is:

• stavi = put (said to one person, informal singular)

So the sentence is giving an instruction.

Related forms:

• stavi = put! (to one person)
• stavite = put! (to more than one person, or formal/polite)

Because this is an instruction, not a description.

• stavi = put! → imperative, used for commands/instructions
• stavljaš = you are putting / you put → present tense, used to describe what someone does

In instructions, Croatian normally uses the imperative:

• Stavi zarez. = Put a comma.
• Otvori knjigu. = Open the book.

So stavi is exactly the expected form here.

Both zarez and točku are direct objects of stavi, so they are in the accusative case.

The difference comes from gender and declension:

• zarez is a masculine inanimate noun
  In the accusative singular, masculine inanimate nouns usually look the same as the nominative:

  • nominative: zarez
  • accusative: zarez

• točka is a feminine noun
  Feminine nouns in -a usually change to -u in the accusative singular:

  • nominative: točka
  • accusative: točku

So:

• stavi zarez
• stavi točku

Here a is close to and, but not exactly the same as English and in every situation.

In Croatian, a often connects two related statements or steps, sometimes with a slight sense of contrast or separation:

• first do this, and then / while / whereas do that

In this sentence:

• Nakon prve rečenice stavi zarez, a na kraju točku.

The a introduces the second instruction:

• put a comma after the first sentence, and put a period at the end

You could think of it as and then or and as for the end. It sounds natural in instructions.

Because in Croatian, a is normally preceded by a comma when it joins two clauses or two coordinated parts like this.

So:

• ..., a ...

This is standard Croatian punctuation.

That often feels different to English learners, because English does not always use commas in exactly the same way. In Croatian, the comma before a is very common and expected here.

Na kraju means at the end.

This is a fixed and very common expression:

• na = on / at
• kraj = end
• kraju = locative singular form of kraj

So grammatically, this is na + locative:

• na kraju = at the end

You can learn it as a chunk, because it is used very often:

• Na kraju knjige... = At the end of the book...
• Na kraju rečenice... = At the end of the sentence...

It certainly could say a na kraju stavi točku, and that would be completely correct.

But Croatian often omits repeated words when they are easy to understand from context. This is called ellipsis.

So:

• stavi zarez, a na kraju točku really means
• stavi zarez, a na kraju stavi točku

The verb stavi is understood from the first clause, so the sentence sounds shorter and more natural.

Yes. In punctuation, točka means the mark .

In English:

• American English usually says period
• British English usually says full stop

Croatian uses točka for that punctuation mark.

Be aware that točka can also mean point or dot in other contexts, depending on the sentence.

Yes, that would also be correct.

Croatian word order is more flexible than English word order. So both of these work:

• Nakon prve rečenice stavi zarez.
• Stavi zarez nakon prve rečenice.

The difference is mainly emphasis:

• Nakon prve rečenice... puts early focus on where
• Stavi zarez... puts early focus on the action

Both are natural.

Yes, you often can.

Both nakon and poslije can mean after.

So these are both understandable:

• nakon prve rečenice
• poslije prve rečenice

A small difference:

• nakon can sound a bit more neutral or formal
• poslije is very common in everyday speech

In an instruction like this, nakon prve rečenice sounds perfectly natural and standard.