Kad dođem kući, osvježit ću se hladnim tušem.

Kad and kada both mean when.

• kad is the shorter, very common everyday form
• kada is a bit fuller and can sound slightly more formal or careful

In this sentence, kad is completely natural. You could also say Kada dođem kući, osvježit ću se hladnim tušem, and the meaning would stay the same.

Because Croatian normally uses the present tense of a perfective verb after time words like kad when talking about a future event.

So:

• Kad dođem kući = When I get home
• not a literal present-time meaning

English does something similar: we say when I get home, not when I will get home.

So even though dođem is formally a present-tense form, the whole clause refers to a future situation.

Dođem is the 1st person singular present form of the verb doći = to come / to arrive.

Its present-tense forms are:

• dođem = I come / I arrive
• dođeš
• dođe
• dođemo
• dođete
• dođu

This verb is a bit irregular, so learners often need to memorize its present forms rather than build them mechanically.

Also, doći is perfective, which is why it fits well here: it refers to a single completed arrival.

Because kući is the form used in the very common expression meaning home after verbs of motion.

So:

• doći kući = to come home
• ići kući = to go home

This is much more natural than using the basic dictionary form kuća.

A useful contrast is:

• kući = home, to home
• kod kuće = at home

So:

• Kad dođem kući = When I get home
• Kad sam kod kuće = When I am at home

Sometimes, but not with exactly the same feel.

• kući usually means home
• u kuću means into the house

So:

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

If you say kući, you are thinking of home as a destination.
If you say u kuću, you are focusing more on physically entering the building.

The dictionary form is osvježiti.

Croatian future tense is often formed with:

• the infinitive
• plus the clitic form of htjeti: ću, ćeš, će, ćemo, ćete, će

So in principle:

• osvježiti + ću

But when the auxiliary comes after the infinitive, the final -i of the infinitive is dropped:

• osvježiti ću → osvježit ću

That is why you see osvježit ću.

Compare:

• Osvježit ću se
• Ja ću se osvježiti

Both are correct; the difference is mainly word order.

Because the verb here is osvježiti se, a reflexive/pronominal verb meaning to refresh oneself or to freshen up.

So:

• osvježiti se = refresh oneself
• osvježiti nešto = refresh something

For example:

• Osvježit ću se = I’ll freshen up
• Osvježio sam piće = I cooled/refreshed the drink

In this sentence, the speaker is doing the action to themselves, so se is needed.

Because Croatian uses the instrumental case here to show the means or method of the action.

So:

• hladnim tušem = with a cold shower / by means of a cold shower

No preposition is needed.

Both words are in the instrumental singular:

• hladan → hladnim
• tuš → tušem

This is a very common use of the instrumental case in Croatian.

Because the instrumental case can often express with / by means of on its own.

So Croatian can say:

• osvježiti se hladnim tušem

literally something like:

• refresh oneself with a cold shower

without needing a separate word for with.

This is one of the important jobs of the instrumental case.

Because ću is a clitic. Clitics in Croatian normally do not stand at the very beginning of their clause.

So a clause usually cannot simply begin with ću.

That is why:

• Osvježit ću se is natural
• Ja ću se osvježiti is also natural
• Ću se osvježiti is not normal

In osvježit ću se, the verb is placed first, and the clitic ću comes right after it.

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

Here, the speaker is clearly I because of:

• dođem = I come
• ću = I will

So ja is unnecessary unless you want emphasis or contrast.

Compare:

• Kad dođem kući, osvježit ću se hladnim tušem. = neutral
• Kad dođem kući, ja ću se osvježiti hladnim tušem. = more emphatic, maybe contrasting with someone else

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

Croatian normally uses a comma in that position:

• Kad dođem kući, osvježit ću se hladnim tušem.

This is similar to English punctuation when an introductory clause comes first.

A rough guide:

• đ in dođem sounds somewhat like the j in jeans, but softer
• ć in kući and ću is a soft palatal sound, somewhere between English t and ch
• š sounds like sh
• ž sounds like the s in measure

So:

• dođem ≈ soft dojem-like sound, but not exactly English
• kući has a soft ć
• ću is not a hard choo sound

These sounds do not match English perfectly, so listening to native audio is very helpful.