Kad sam umorna, pokušavam se odmarati što više.

Because the adjective has to agree with the speaker.

Here the subject is an implied ja = I, and umorna shows that the speaker is feminine.
If a man said the same sentence, it would be:

Kad sam umoran, pokušavam se odmarati što više.

So:

• umorna = feminine singular
• umoran = masculine singular

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

Here, sam and pokušavam clearly indicate I:

• sam = I am
• pokušavam = I try / I am trying

So ja is unnecessary unless you want emphasis or contrast.

For example:

• Kad sam umorna, pokušavam se odmarati što više. = neutral
• Kad sam umorna, ja pokušavam se odmarati što više. = more emphatic, like I try to rest as much as possible

Here kad means when.

Yes, kad and kada usually mean the same thing. The difference is mostly style:

• kad = more common in everyday speech
• kada = a bit fuller, sometimes slightly more formal

So both are possible:

• Kad sam umorna...
• Kada sam umorna...

Sam is the 1st person singular present form of biti = to be.

So:

• sam = am

The phrase sam umorna literally means am tired, and with the implied subject ja, it means I am tired.

This is a very common Croatian structure:

• Ja sam sretna. = I am happy.
• Ja sam umorna. = I am tired.

Because pokušavam is imperfective, and it fits a habitual or ongoing meaning better.

The sentence describes something you generally do whenever you are tired, not one single completed attempt.

• pokušavam = I try / I keep trying
• pokušam = I try once / I make an attempt

So in this sentence, pokušavam sounds more natural because it expresses a repeated or usual action.

Because the verb is odmarati se, not just odmarati.

Odmarati se means to rest.
The se is part of the verb and is often not translated separately into English.

This is common in Croatian. Some verbs simply require se:

• odmarati se = to rest
• smijati se = to laugh
• nadati se = to hope

So if you remove se, the sentence would no longer be correct for this meaning.

Because se is a clitic, and clitics move to certain positions in a sentence.

The dictionary form is:

• odmarati se

But when another verb comes before the infinitive, Croatian usually places se before the infinitive:

• pokušavam se odmarati
• želim se odmarati
• moram se odmarati

So the dictionary form stays odmarati se, but inside a sentence you often see se moved forward.

This is a question of aspect.

• odmarati se = imperfective, ongoing or repeated resting
• odmoriti se = perfective, to get rested / to rest up in one complete event

In this sentence, the speaker means a general, ongoing habit: when tired, they try to rest as much as possible. That is why the imperfective odmarati se is natural.

Compare:

• Pokušavam se odmarati što više. = I try to rest as much as possible.
• Pokušavam se odmoriti. = I’m trying to get some rest / to recover by resting.

Što više means as much as possible or literally as much/more as possible in this kind of sentence.

It is a common Croatian expression:

• što više = as much/as many as possible
• što manje = as little/as few as possible

So here it tells you the speaker wants to maximize the amount of rest.

A very similar sentence would be:

• Pijem vodu što više mogu. = I drink as much water as I can.

Yes. The comma is standard here because Kad sam umorna is a subordinate clause placed before the main clause.

So the structure is:

• subordinate clause: Kad sam umorna
• main clause: pokušavam se odmarati što više

Croatian normally uses a comma between them in this order.

Yes, Croatian word order is fairly flexible, but not completely free.

A very natural alternative is:

Kad sam umorna, pokušavam se što više odmarati.

This puts što više closer to odmarati, but the meaning stays the same.

What you should be careful about is se, because clitics have stricter placement rules. So while some rearranging is possible, not every version will sound natural or correct.

Yes. Umorna is an adjective meaning tired, and here it is used predicatively, after the verb biti.

So the structure is:

• sam = am
• umorna = tired

Together: sam umorna = am tired

This is exactly like many other Croatian sentences with biti + adjective:

• Sam gladna. = I am hungry.
• Sam sretna. = I am happy.
• Sam umorna. = I am tired.