Kad je moja mama umorna, pije čaj u kuhinji.

Kad means when. Here it introduces the time clause when my mother is tired.

Yes, you can also say kada. Both are correct:

• Kad je moja mama umorna...
• Kada je moja mama umorna...

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

Because je is a clitic, an unstressed form of the verb biti (to be). In Croatian, clitics usually appear very early in the clause, often in the second position.

So Croatian prefers:

• Kad je moja mama umorna

not:

• Kad moja mama je umorna

If you say the statement by itself, you get:

• Moja mama je umorna.

That is the normal word order.

Je is the 3rd person singular present tense of biti (to be).

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In this sentence, je means is:

• moja mama je umorna = my mother is tired

Because they agree with mama, which is a feminine singular noun.

In Croatian, possessives and adjectives agree with the noun in gender, number, and case.

So:

• moja mama = my mother
  not moj mama
• mama je umorna = mother is tired
  not umoran

Compare:

• moj tata je umoran = my dad is tired
• moja mama je umorna = my mom is tired

Croatian often leaves out subject pronouns when they are already clear from the context. This is very normal.

So instead of saying:

• Ona pije čaj u kuhinji

Croatian can simply say:

• Pije čaj u kuhinji

In this sentence, we already know the subject is moja mama, so ona is unnecessary.

You can add ona if you want extra emphasis, but it is not needed.

Pije is the 3rd person singular present tense of piti (to drink).

So:

• ja pijem = I drink
• ti piješ = you drink
• on/ona pije = he/she drinks

Here pije matches the subject moja mama, which is she.

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

• piti / pije = to drink, imperfective
• popiti / popije = to drink up, finish drinking, perfective

In this sentence, the meaning is general or habitual: when she is tired, she drinks tea in the kitchen. For that kind of repeated or ongoing action, Croatian normally uses the imperfective verb:

• pije

If you said popije, it would suggest something more like she drinks up / finishes a tea, often as a single completed action.

Because u can take different cases depending on the meaning:

• u + locative = location, in
• u + accusative = movement toward, into

Here the meaning is location:

• u kuhinji = in the kitchen

She is already there while drinking tea, so Croatian uses the locative.

Compare:

• Pije čaj u kuhinji. = She drinks tea in the kitchen.
• Ide u kuhinju. = She goes into the kitchen.

The noun kuhinja changes like this:

• nominative: kuhinja
• locative: kuhinji
• accusative: kuhinju

Because Croatian does not have articles like English a/an and the.

So Croatian simply says:

• mama
• čaj
• kuhinji

The listener understands from context whether something is definite or indefinite.

That is why pije čaj can mean drinks tea, and u kuhinji can mean in the kitchen, without any separate article word.

Because Kad je moja mama umorna is a subordinate clause, and it comes before the main clause.

Croatian normally uses a comma to separate that kind of clause from the main clause:

• Kad je moja mama umorna, pije čaj u kuhinji.

This is similar to English:

• When my mother is tired, she drinks tea in the kitchen.

Yes, Croatian word order is fairly flexible, but some versions sound more neutral than others.

The most neutral version here is:

• pije čaj u kuhinji

You could also say:

• ona pije čaj u kuhinji
• pije u kuhinji čaj
• u kuhinji pije čaj

These can be grammatical, but they may sound more marked or emphasize a different part of the sentence.

So for a learner, pije čaj u kuhinji is the safest and most natural basic order.

A few sounds here are especially useful for English speakers:

• č in čaj sounds roughly like ch
• j sounds like English y
• so čaj sounds close to chai

In kuhinji:

• j again sounds like y
• nj sounds like the ny sound in canyon

So kuhinji has a sound roughly like koo-HIN-yee, though the exact Croatian pronunciation is a bit different from English.

Also, Croatian spelling is very regular, so once you learn the sound values of letters, pronunciation becomes much easier.