Kad padne mrak, idemo kući.

Kad means when. It is the shorter, very common form of kada.

In most everyday Croatian, kad and kada mean the same thing. The difference is mainly style:

• kad = shorter, more conversational
• kada = a bit more formal or more emphatic

So both of these work:

• Kad padne mrak, idemo kući.
• Kada padne mrak, idemo kući.

Because padne is the perfective form, and Croatian often uses a perfective present after kad to talk about a future or completed event in a time clause.

Here:

• padne = falls / comes down in the sense of once darkness falls
• pada would describe an ongoing process: is falling

So:

• Kad padne mrak = when it gets dark / when darkness falls
• Kad pada mrak would sound more like while it is getting dark or whenever darkness is falling, which is a different idea

This is a very common Croatian pattern:

• Kad dođeš, nazovi me. = When you come, call me.
• Kad završim, idem kući. = When I finish, I’m going home.

Padne is the 3rd person singular present form of the verb pasti.

Breakdown:

• pasti = to fall
• padne = he/she/it falls or, in this kind of sentence, when it falls / when it comes

In padne mrak, the subject is mrak, so literally it is something like:

• darkness falls

Croatian uses this kind of expression where English often prefers:

• it gets dark
• night falls

Because mrak is the subject of the verb padne.

The structure is literally:

• mrak padne = darkness falls

So mrak is not an object like darkness in I see darkness. It is the thing doing the action grammatically.

Croatian often allows flexible word order, so even though padne comes before mrak, mrak is still the subject.

Croatian often uses the present tense where English might use either:

• a habitual present: we go home
• a planned/near-future present: we’re going home

So idemo kući can mean:

• we go home regularly
• we’re going home as a plan

In this sentence, the meaning depends on context. It can be:

• a general rule: When it gets dark, we go home
• a current plan: When it gets dark, we’re going home

If you want to make the future more explicit, you can say:

• Kad padne mrak, ići ćemo kući. = When it gets dark, we’ll go home.

Also, after kad, Croatian normally does not use future tense the way English sometimes does. So Croatian prefers:

• Kad padne mrak... not
• Kad će pasti mrak...

Because Croatian usually drops subject pronouns when they are not needed.

The ending of the verb already tells you who the subject is:

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

So idemo already means we go / we’re going.

You can add mi for emphasis or contrast:

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

Here kući means home, specifically to home / homeward after a verb of motion.

So:

• ići kući = to go home

Historically, kući is related to the noun kuća = house, but in this expression it works like a fixed form meaning home.

This is why Croatian says:

• Idem kući. = I’m going home.

and not normally:

• Idem kuću.

A useful contrast:

• kuća = house
• u kuću = into the house
• kući = home, to home

This is a very common and important distinction:

• kući = homeward / to home
  Used with movement:

  • Idem kući. = I’m going home.

• kod kuće = at home
  Used for location:

  • Kod kuće sam. = I’m at home.

So:

• Idemo kući. = We’re going home.
• Mi smo kod kuće. = We’re at home.

English uses home for both ideas, but Croatian separates them more clearly.

Because Kad padne mrak is a subordinate time clause placed at the beginning of the sentence, and it is normally separated from the main clause by a comma.

So:

• Kad padne mrak, idemo kući.

If you reverse the order, the sentence is also natural:

• Idemo kući kad padne mrak.

When the kad clause comes first, the comma is especially expected.

Yes. Croatian word order is fairly flexible.

The most neutral version here is:

• Kad padne mrak, idemo kući.

But you can also say:

• Idemo kući kad padne mrak.

That means the same thing, just with a different focus.

You may also see:

• Kad mrak padne, idemo kući.

This is understandable, but it sounds more marked, poetic, or literary than the usual Kad padne mrak.

It is not completely fixed, but it is a very common natural expression.

It literally means:

• when darkness falls

English often uses different wording, such as:

• when it gets dark
• when night falls

Croatian can express the same idea in several natural ways:

• Kad padne mrak...
• Kad padne noć...
• Kad se smrači...

All are possible, but Kad padne mrak is a normal, idiomatic way to say it.

A very literal breakdown is:

• Kad = when
• padne = falls
• mrak = darkness
• idemo = we go / we’re going
• kući = home

So the literal structure is:

• When darkness falls, we go home.

That literal picture is helpful, because it shows how Croatian builds the sentence:

• first the time clause
• then the main action

It is a good example of how Croatian often says things a bit differently from English, while still sounding completely natural.