Kako je već kasno, idemo kući.

In this sentence, kako is not a question word. It is being used as a conjunction introducing a reason:

• Kako je već kasno, idemo kući. = Since it is already late, we are going home.

This use of kako is correct, but it can sound a bit more literary or formal than some other options. In everyday Croatian, learners will often hear:

• Idemo kući jer je već kasno.
• Budući da je već kasno, idemo kući.
• Pošto je već kasno, idemo kući.

So yes, kako usually means how, but in sentences like this it can also mean since / as.

Croatian often uses impersonal expressions where English would use a dummy subject like it.

• Kasno je. = It is late.
• Hladno je. = It is cold.
• Vruće je. = It is hot.

English needs it, but Croatian does not. So je is simply the 3rd person singular form of biti = to be, and the subject is not stated because Croatian does not need one here.

Here kasno is the form Croatian uses in expressions like it is late.

Compare:

• On je kasan. = He is late.
• Ona je kasna. = She is late.
• Kasno je. = It is late.

So in impersonal time expressions, Croatian normally uses kasno. You will see the same pattern with words like:

• rano je = it is early
• hladno je = it is cold
• mračno je = it is dark

Već usually means already.

So:

• Kasno je. = It is late.
• Već je kasno. = It is already late.

It adds the idea that the time has reached that point by now, sometimes with a slight sense of later than expected or later than is convenient.

Because Kako je već kasno is a subordinate clause placed before the main clause.

Structure:

• Kako je već kasno, = Since it is already late,
• idemo kući. = we are going home.

In Croatian, when this kind of dependent clause comes first, it is normally separated from the main clause by a comma.

Because Croatian verb endings usually show the subject clearly.

• idem = I go
• ideš = you go
• ide = he/she/it goes
• idemo = we go

So idemo already means we go / we are going, and mi is not necessary.

You could say mi idemo kući, but that would usually add emphasis, contrast, or clarity:

• Mi idemo kući, a oni ostaju. = We are going home, and they are staying.

Kući is a very common fixed form meaning home after verbs of motion:

• Idem kući. = I am going home.
• Vraćamo se kući. = We are returning home.

This is different from u kuću, which means into the house as a physical building.

So:

• Idemo kući. = We are going home.
• Idemo u kuću. = We are going into the house.

A good way to remember it is that kući often behaves like the English word home, not like the ordinary noun house.

Yes, but the sentence structure would usually change.

Most natural:

• Idemo kući jer je već kasno. = We are going home because it is already late.

Also natural:

• Budući da je već kasno, idemo kući.
• Pošto je već kasno, idemo kući.

Using jer at the beginning like Jer je već kasno, idemo kući is much less natural in standard usage. With jer, the reason clause usually comes after the main clause.

The sentence is correct and natural, but kako in this causal meaning can sound a bit more written, formal, or literary than the most everyday option.

Very common conversational alternatives are:

• Idemo kući jer je već kasno.
• Pošto je već kasno, idemo kući.

So a learner should understand Kako je već kasno, idemo kući, but in ordinary speech they may more often hear jer or pošto.