Kad citiraš nečije riječi, stavi ih u navodnike i pazi gdje ide zarez.

They both mean when. Kad is the shorter, very common everyday form, while kada is a bit fuller and can sound slightly more formal or careful.

In this sentence, kad has a general sense, almost like whenever: whenever you quote someone’s words, do these things.

Croatian often leaves subject pronouns out because the verb ending already shows who the subject is.

Here:

• citiraš = you quote
• stavi = put
• pazi = watch out / be careful

You could add ti for emphasis, but in a neutral instruction it is normally omitted.

Citiraš is the 2nd person singular present form of citirati.

So grammatically it means you quote. In this sentence it is used in a general instruction, not to talk about one specific moment. Croatian often uses the present tense like this after kad in rules, instructions, and general truths.

Nečije means someone’s or somebody else’s.

It comes from nečiji, a possessive word meaning belonging to someone. Its form changes to match the noun it describes. Here it matches riječi, so it appears as nečije riječi.

Riječi is the plural of riječ, meaning word.

After citirati, the thing being quoted is the direct object, so it is in the accusative. For this noun, the nominative plural and accusative plural look the same: riječi. So even though it is the object, the form does not change visibly.

Because these are imperatives, used for instructions or commands.

• stavi = put
• pazi = watch out / be careful

There is also a useful aspect difference here:

• staviti is perfective, so stavi suggests a completed action: put them in quotation marks
• paziti is imperfective, so pazi suggests ongoing attention: keep an eye on where the comma goes

Ih means them, and here it refers to riječi.

It is a short unstressed object pronoun, very common in Croatian. Croatian often uses these short pronouns instead of repeating the noun. So instead of repeating riječi, the sentence says stavi ih u navodnike.

Its position is also normal: after an imperative, stavi ih is the natural order.

This is a very common Croatian pattern with u:

• u + accusative = movement into something
• u + locative = location inside something

Here the idea is put them into quotation marks, so Croatian uses u navodnike.

Compare:

• Stavi ih u navodnike = put them in quotation marks
• Oni su u navodnicima = they are in quotation marks

Also, navodnici is plural because quotation marks are thought of as a pair.

The comma after riječi separates the kad-clause from the main clause:

Kad citiraš nečije riječi, ...

That is standard Croatian punctuation with an introductory subordinate clause.

There is no comma before i because i simply joins two coordinated commands:

• stavi ih u navodnike
• pazi gdje ide zarez

In a normal sentence like this, no comma is needed before i.

That is a natural way to talk about punctuation in Croatian. The verb ići often means not only to go, but also to belong / to be placed in expressions like this.

So gdje ide zarez means where the comma should be placed.

Also, zarez is singular because the sentence is talking about comma placement in general, or about the relevant comma in each quoted sentence.

To some extent, yes. Croatian word order is more flexible than English word order because endings carry a lot of grammatical information.

For example, you could also say:

Kad citiraš nečije riječi, pazi gdje ide zarez i stavi ih u navodnike.

That still works. But word order is not completely free. Short pronouns like ih have placement rules, so the original stavi ih is much more natural than something like ih stavi in a neutral sentence.