Njih troje često zajedno doručkuje prije nego što krenu u vrtić i školu, a blizanci uvijek sjede jedan pokraj drugoga.

Croatian has special collective numerals such as dvoje, troje, četvero. They are very commonly used for children and for mixed groups of people.

So njih troje means the three of them.

The form njih is used because this structure is basically partitive: three of them, not simple nominative they. That is why njih troje is natural here, not oni troje.

Useful comparison:

• njih trojica = the three of them, all male
• njih tri = the three of them, all female
• njih troje = very natural for children or a mixed group

Because troje behaves like a collective subject. With subjects like dvoje, troje, četvero, Croatian often uses a singular verb, treating the group as one unit.

So:

• Njih troje doručkuje = normal
• literally, the group of three is being viewed together

If this were in the past tense, you could see this even more clearly:

• Njih troje je doručkovalo.

Later in the sentence you get krenu, which is plural. That is because the subject there is understood more as they as individual people, not as one grouped unit.

Here prije nego što is a fixed conjunction meaning before when it introduces a full clause.

So:

• prije nego što krenu = before they leave / before they set off

In this expression, što does not mean what in the usual question-word sense. It is just part of the conjunction.

This is the safest standard pattern to learn:

• prije nego što + finite verb

Krenu is the 3rd person plural present form of krenuti.

krenuti means to set off, to start out, to leave, so it focuses on the moment they begin going somewhere.

That is a bit different from ići, which is more general:

• idu = they go / they are going
• krenu = they set off / they head out

Also, after time expressions like prije nego što, Croatian often uses a present-tense form even when English would think of a future-related action:

• prije nego što krenu = before they leave

Yes. The sentence uses aspect very naturally.

• doručkuje comes from an imperfective verb and describes a repeated, habitual activity: they often have breakfast together.
• krenu comes from a perfective verb and refers to a single complete action each time: they set off.

So the contrast is:

• habitual background activity: često zajedno doručkuje
• one bounded action that follows: prije nego što krenu

This is very typical Croatian aspect usage.

Because u with a destination takes the accusative case.

So:

• u vrtić
• u školu

Both vrtić and školu are in the accusative because the sentence is about movement toward a place.

Croatian often does not repeat the same preposition when it applies to two coordinated nouns, so:

• u vrtić i školu = perfectly normal

You could also say u vrtić i u školu, but repeating u is not necessary here.

Also, this does not have to mean that every child goes to both places. It can simply mean that, as a group, they head off to kindergarten and school respectively.

A often sits somewhere between and, while, and whereas.

In this sentence, it introduces a new but connected piece of information:

• first clause: what the three children do before leaving
• second clause: what the twins specifically do

So here a is not a strong but. It is more like:

• and
• while
• whereas, depending on the context

The comma before a is standard when it joins two independent clauses.

Because the second clause is no longer talking about all three children together. It is zooming in on two of them: the twins.

Blizanci means twins.

So the structure is:

• njih troje = all three children
• blizanci = the two twins within that group

This is a very natural way to narrow the focus in Croatian.

Yes, at least partly.

Blizanci is a masculine plural form. It is used:

• for male twins
• often as the default form when gender is mixed or not specially emphasized

If the twins were specifically female, you would normally expect:

• blizanke

So blizanci suggests either male twins or an unmarked/default reference.

It means next to each other or literally one beside the other.

This is a common Croatian reciprocal pattern:

• jedan ... drugoga

Even though English uses each other, Croatian often uses forms meaning one ... the other.

Here:

• jedan = one
• pokraj = beside / next to
• drugoga = the other one

So sjede jedan pokraj drugoga means they sit next to each other.

Very similar expressions are:

• jedan do drugoga
• jedan pored drugoga

Because pokraj requires the genitive case.

So drugi changes to drugoga here.

That is why you get:

• jedan pokraj drugoga

You may also hear the shorter form drugog in everyday speech, but drugoga is a very normal standard form.

The masculine form is used because the sentence refers to blizanci. If the referent were feminine, the form would change accordingly.

Not completely. Croatian word order is fairly flexible, especially with adverbs.

This sentence uses a very natural neutral order:

• često zajedno doručkuje
• uvijek sjede

But you could move adverbs around for emphasis:

• Njih troje zajedno često doručkuje
• Blizanci sjede uvijek jedan pokraj drugoga

Those alternatives are possible, but the original sounds more natural and neutral.

So the chosen word order is mainly about normal flow and emphasis, not a different basic meaning.