Learn Hardcore Croatian: Kad djeca uče voziti bicikl, učitelj ih vodi do semafora i pokazuje kako staviti kacigu. - When children are learning to ride a bicycle, the teacher leads them to the traffic light and shows them how to put on a helmet.

Kad and kada have the same meaning.
Kad is the shorter, more colloquial form and is extremely common in speech and informal writing.
Kada can sound a bit more formal or emphatic, but in most contexts they are interchangeable:
Kad djeca uče voziti bicikl…
Kada djeca uče voziti bicikl…

Both are correct: “When children learn to ride a bicycle…”

Why is djeca used with the plural verb uče, even though djeca looks like a singular word?

Djeca means “children” and is grammatically plural, even though it doesn’t end in a typical plural ending.

djete / dijete = child (singular)
djeca = children (plural)

That’s why the verb is in the 3rd person plural:

Djeca uče. – The children learn / are learning.
Kad djeca uče voziti bicikl… – When children are learning to ride a bike…

So: djeca behaves like they in English, not like it.

What exactly is the structure uče voziti bicikl? Why are there two verbs in a row?

This is the pattern učiti + infinitive, which corresponds to “learn to do something” in English.

učiti = to learn, to study (imperfective)
voziti = to ride/drive (infinitive)
bicikl = bicycle (accusative singular)

So uče voziti bicikl literally means “they are learning to ride a bicycle”.

This pattern is very common:

učim govoriti hrvatski – I’m learning to speak Croatian
uči plivati – (He/She) is learning to swim

You may sometimes see učiti se voziti bicikl, adding se, but here uče voziti bicikl is perfectly natural and common.

Why is it voziti bicikl and not something with a preposition, like voziti na biciklu?

In Croatian, voziti bicikl is the standard way to say “to ride a bike”:

voziti is a transitive verb here and takes a direct object in the accusative:
- voziti auto – to drive a car
- voziti bicikl – to ride a bicycle

Saying voziti na biciklu would literally be “to drive on a bicycle”, which is not how people normally express “riding a bike” in Croatian. The natural collocation is voziti bicikl.

What does učitelj mean exactly? Is it a specific kind of teacher?

Učitelj means “teacher”, but it most typically refers to:

a primary/elementary school teacher, or
a teacher/instructor in a non-school setting (e.g. someone teaching you to ride a bike).

Other common words:

nastavnik – usually a subject teacher in middle/high school
profesor – a high school teacher (especially gymnasium) or a university professor

In this sentence, učitelj is a general instructor/teacher showing children traffic rules and safety.

What does the pronoun ih refer to, and which case is it?

Ih refers to djeca (the children) and is the 3rd person plural accusative pronoun (“them”).

učitelj ih vodi do semafora
Literally: “the teacher them leads to the traffic light.”

Key points:

ih = them (direct object, accusative).
It’s a clitic (short unstressed form) and normally sits in the second position in the clause, so it comes right after učitelj:
- Učitelj ih vodi…, not Učitelj vodi ih… in neutral word order.

You could also say:

Učitelj vodi djecu do semafora. – The teacher leads the children to the traffic light.

Both are correct; the original just uses a pronoun instead of repeating djeca.

Why is it do semafora and not do semafor or do semaforom?

The preposition do (“to / up to / as far as”) always takes the genitive case.

Nominative (dictionary form): semafor
Genitive singular: semafora

So:

do + genitive → do semafora – “to the traffic light”

Other examples:

do kuće – to the house
do škole – to the school
do auta – to the car

Forms like do semafor or do semaforom are ungrammatical.

What does semafor mean exactly? Is it only for car traffic lights or also for pedestrian lights?

Semafor means a traffic light in general: the device with red/yellow/green lights controlling traffic.

It can refer to:

traffic lights for cars,
pedestrian crossings,
or a junction with traffic lights.

So in this sentence, do semafora simply means “to the traffic light(s)”, regardless of whether they are for cars or pedestrians.

In pokazuje kako staviti kacigu, why is staviti an infinitive and not a conjugated form like stavi or stavljaju?

The pattern pokazati / pokazivati kako + infinitive is used to express “show how to do something”:

pokazuje kako staviti kacigu – shows (them) how to put on a helmet

Here:

pokazuje – he/she shows (imperfective)
kako staviti – how to put on (infinitive)
kacigu – a helmet (accusative)

You could also say:

pokazuje im kako da stave kacigu – shows them how they should put on the helmet

But kako + infinitive is shorter and very common.

Why is kacigu ending in -u and not -a like the dictionary form kaciga?

Kaciga (helmet) is a feminine noun in -a. In this sentence it is the direct object of staviti, so it appears in the accusative singular:

Nominative: kaciga – a helmet (subject)
Accusative: kacigu – a helmet (object)

Pattern for many feminine -a nouns:

kaciga → kacigu
torba → torbu (bag)
knjiga → knjigu (book)

So staviti kacigu = “to put on a helmet.”

Could the sentence be reordered, for example: Učitelj vodi djecu do semafora kad uče voziti bicikl? Would the meaning change?

You can reorder it, and the basic meaning stays the same, but the focus shifts slightly.

Original:

Kad djeca uče voziti bicikl, učitelj ih vodi do semafora i pokazuje kako staviti kacigu.
Focus: when children are learning to ride, then the teacher does these things.

Alternative:

Učitelj vodi djecu do semafora kad uče voziti bicikl.
Focus: what the teacher does (leads them to the traffic light) when they are learning.

Both are grammatical. Croatian word order is quite flexible, but the original order Kad … , ... is a very typical way to start a sentence with a time clause.

Why is there a comma after bicikl?

The comma separates a subordinate clause introduced by kad from the main clause.

Subordinate clause (time): Kad djeca uče voziti bicikl – When children are learning to ride a bicycle
Main clause: učitelj ih vodi do semafora i pokazuje kako staviti kacigu – the teacher leads them to the traffic light and shows how to put on a helmet.

Rule: in Croatian, a clause introduced by kad (when), ako (if), jer (because), etc. is usually separated from the main clause by a comma when they are in this order:

Kad …, (then) …
Ako …, (then) …

So the comma after bicikl is required by this clause structure.