Kad dođem kući, želim se malo odmoriti.

Yes. Kad and kada both mean when.

• kad is the shorter, very common everyday form
• kada is a bit more formal or more emphatic

So Kad dođem kući... and Kada dođem kući... both work here.

In Croatian, after time words like kad meaning when, you often use the present tense to talk about a future event.

So:

• Kad dođem kući = When I get home

This is actually similar to English, which also says when I get home, not when I will get home.

This is about aspect.

• doći / dođem is perfective: it focuses on a single completed arrival
• dolaziti / dolazim is imperfective: it focuses on repeated, ongoing, or habitual coming

Here the speaker means one future moment of arrival: first I get home, then I rest. That is why dođem is the natural choice.

Not in this sentence.

In Kad dođem kući, kad introduces a time clause meaning when / once.

But kad ću doći kući usually means when I will come home as an indirect question, for example:

• Ne znam kad ću doći kući. = I don’t know when I’ll come home.

So in your sentence, kad dođem kući is correct, not kad ću doći kući.

Because kući is the usual idiomatic way to say home after verbs of motion:

• ići kući = go home
• doći kući = come home
• vratiti se kući = return home

If you say u kuću, that means into the house and focuses on the physical building.

So:

• doći kući = arrive home
• doći u kuću = come into the house

Formally, kući is an old case form of kuća. In modern Croatian, though, learners usually do best to treat kući as a fixed expression meaning home in phrases like ići kući and doći kući.

So the most useful thing to remember is not the label, but the pattern:

• doći kući = come home

Because Croatian usually leaves out subject pronouns when the verb already shows who the subject is.

Here:

• dođem already means I come / I arrive
• želim already means I want

So ja is not necessary. You could add it for emphasis, but the neutral version normally leaves it out.

Because Croatian commonly uses željeti + infinitive to say want to do something.

So:

• želim se odmoriti = I want to rest

That is the normal standard Croatian pattern here.

You may hear other structures in the wider region, but for standard Croatian, želim + infinitive is the best model to learn.

The verb is odmoriti se, which means to rest or to get some rest.

The se is important. Without it, odmoriti usually means to rest something/someone or give rest to something, for example:

• odmoriti noge = rest your legs

But:

• odmoriti se = rest

So se is part of the verb you need here.

Because se is a clitic: a short unstressed word that usually goes very early in the clause, often in the second position.

So Croatian prefers:

• Želim se malo odmoriti.

Not:

• Želim malo odmoriti se.

Even though se belongs in meaning with odmoriti, its position follows clitic placement rules.

Again, this is aspect.

• odmoriti se = perfective: to rest up, to have a rest, one complete action
• odmarati se = imperfective: to be resting, to rest repeatedly, or to focus on the process

Here the speaker means a single future action: when I get home, I want to rest a bit. So odmoriti se is the natural choice.

Yes, literally a little, but in English the most natural translation is usually:

• a bit
• for a little while

So:

• želim se malo odmoriti = I want to rest a bit

It makes the sentence sound softer and more natural.

Because Kad dođem kući is a subordinate time clause placed before the main clause.

So the sentence is divided like this:

• Kad dođem kući, = time clause
• želim se malo odmoriti. = main clause

That is why a comma appears after kući.

Yes. Croatian word order is fairly flexible, but the emphasis changes.

For example, you can also say:

• Želim se malo odmoriti kad dođem kući.

This means the same thing overall.

The version with Kad dođem kući first gives the time setting first. The version with Želim se malo odmoriti first emphasizes the wish/desire first.

But the placement of se still has to follow clitic rules, so it cannot just go anywhere.