I kad sam umoran, pokušavam ponoviti nove riječi.

Yes, i very often means and, but in this sentence i kad means even when.

So:

• i = adds emphasis here
• kad = when

Together, I kad sam umoran means Even when I am tired.

This is a very common use of i in Croatian:

• i ako... = even if...
• i kad... = even when...

Because Croatian often leaves out subject pronouns when they are already clear from the verb form.

• pokušavam = I try / I am trying

The ending -am shows first person singular, so the pronoun ja is not necessary.

You could say Ja pokušavam..., but that would usually add extra emphasis, something like I am the one who tries...

There are two things going on here:

1. ja is usually omitted, because sam already tells you it is I am
2. sam is a clitic, and clitics usually go in the second position in the clause

So Croatian prefers:

• kad sam umoran

not:

• kad ja sam umoran

If you do include ja, the normal order would be:

• kad sam ja umoran

That sounds more emphatic: when I am tired

Yes. The adjective changes for gender.

• umoran = used by a male speaker
• umorna = used by a female speaker

So:

• I kad sam umoran, pokušavam ponoviti nove riječi. = said by a man
• I kad sam umorna, pokušavam ponoviti nove riječi. = said by a woman

Croatian adjectives usually agree with the gender of the person they describe.

Because kad sam umoran is a subordinate clause, and it comes before the main clause.

So the structure is:

• I kad sam umoran, = subordinate clause
• pokušavam ponoviti nove riječi. = main clause

Croatian normally separates this kind of introductory clause with a comma.

This is about aspect, which is very important in Croatian verbs.

• ponoviti = perfective
• ponavljati = imperfective

Here, pokušavam ponoviti means something like I try to repeat / manage to repeat the new words, focusing on the action as a whole.

If you said pokušavam ponavljati, it would sound more like I try to keep repeating or I try to do repetition as an ongoing activity.

So ponoviti is natural here because the idea is trying to complete the act of repeating.

Nove riječi is in the accusative plural, because it is the direct object of ponoviti.

• ponoviti što? = repeat what?
• nove riječi = new words

For this noun, the accusative plural looks the same as the nominative plural:

• nove riječi = nominative plural
• nove riječi = accusative plural

So the form does not change here, even though the case function does.

Because riječ is an irregular feminine noun.

Its plural is:

• singular: riječ = word
• plural: riječi = words

This is just the correct plural form you need to learn. Many feminine nouns do not form the plural in exactly the same way, so riječ → riječi is a vocabulary item worth memorizing.

Yes. Kad and kada both mean when.

In this sentence, both are possible:

• I kad sam umoran...
• I kada sam umoran...

The shorter kad is very common in everyday speech.
Kada can sound a little more formal or slightly more careful, but the meaning is the same here.

It is the present tense, and depending on context it can mean either:

• I try
• I am trying

In this sentence, it most naturally sounds like a habitual present:

• Even when I am tired, I try to repeat new words

So it describes something the speaker usually does, not necessarily only something happening right now.

A few sounds may be tricky for English speakers:

• š sounds like sh in shoe
• č sounds roughly like ch in church, but a bit firmer
• j sounds like English y in yes

So roughly:

• pokušavam ≈ poo-KOO-sha-vam
• riječi ≈ RYE-chee

Also:

• rije at the start of riječi is one syllable sequence, not like English ree-yeh
• Croatian spelling is quite regular, so once you know the letters, pronunciation becomes much easier