Learn Hardcore Croatian: On radi u gradu, a ja učim hrvatski kod kuće. - He works in the city, and I study Croatian at home.

In Croatian, subject pronouns (ja, ti, on, ona, mi, vi, oni…) are usually dropped because the verb ending already shows who is doing the action.

Radi u gradu. = He/She works in the city.
On radi u gradu. = He works in the city.

Using on makes the subject explicit and emphasized. In this sentence, on contrasts with ja:

On radi u gradu, a ja učim hrvatski kod kuće.
He works in the city, but I study Croatian at home.

So the pronoun is not grammatically necessary, but it is natural here because of the contrast he vs I.

What verb is radi from, and how is it conjugated?

Radi comes from the infinitive raditi (to work, to do).

Present tense of raditi:

ja radim – I work
ti radiš – you (sg.) work
on/ona/ono radi – he/she/it works
mi radimo – we work
vi radite – you (pl./formal) work
oni/one/ona rade – they work

So on radi literally means he works.

Why is it u gradu and not u grad?

Because u can take different cases depending on meaning:

u
- accusative = into (movement to a place)
u
- locative = in, inside (position in a place)

Here we talk about location, not movement:

u gradu = in the city (locative)
u grad = (into) the city (accusative, usually with a verb of motion like ići – to go)

So On radi u gradu means He works *in the city* (he is already there).

What case is gradu, and why does it look like that?

Gradu is locative singular of grad (city, town).

Declension of grad (masculine noun) in singular:

Nominative: grad – the city
Genitive: grada – of the city
Dative: gradu – to/for the city
Accusative: grad – (into) the city
Locative: gradu – in the city
Instrumental: gradom – with the city

After the preposition u with the meaning in, you use the locative, so u gradu.

What is the difference between a, i, and ali? Why do we say a ja, not i ja or ali ja?

All three are conjunctions, but they have different nuances:

i = and (simply adds information, no contrast)
- On radi u gradu i uči hrvatski. – He works in the city and studies Croatian.
ali = but (strong opposition)
- On radi u gradu, ali ja ne radim. – He works in the city, but I don’t.
a = and / while / whereas / but (mild contrast or comparison)
- On radi u gradu, a ja učim hrvatski kod kuće.
  He works in the city, while/whereas I study Croatian at home.

Here a is perfect because we are contrasting what he does with what I do, but not in a strongly opposing way; it’s more like he does X, I do Y.

Why is there a comma before a?

In Croatian, there is normally a comma before coordinating conjunctions like a, ali, nego, već, especially when they join two independent clauses.

On radi u gradu, a ja učim hrvatski kod kuće.

Both parts are full sentences:

On radi u gradu.
Ja učim hrvatski kod kuće.

Because of that, you put a comma before a.

What verb is učim from, and how is it conjugated?

Učim comes from the infinitive učiti (to study, to learn).

Present tense of učiti:

ja učim – I study / learn
ti učiš – you (sg.) study / learn
on/ona/ono uči – he/she/it studies / learns
mi učimo – we study / learn
vi učite – you (pl./formal) study / learn
oni/one/ona uče – they study / learn

So ja učim hrvatski = I study Croatian or I am learning Croatian.

Why do we say učim hrvatski, not učim hrvatski jezik?

Both are correct:

Učim hrvatski.
Učim hrvatski jezik.

Hrvatski is originally an adjective (Croatian), but it is very common to use it as a noun meaning the Croatian language. The full phrase is hrvatski jezik, but jezik (language, tongue) is often dropped in everyday speech when it’s clear from context.

So učim hrvatski is completely natural and very common.

What form of hrvatski is this? Why does it end in -i?

Hrvatski here is:

an adjective originally (Croatian),
used as a noun,
in the accusative singular masculine.

Masculine animate nouns and adjectives often have the same form in accusative and genitive, ending in -og/-og(a) or -i in some patterns, but with adjectives like this in everyday speech the form hrvatski is used for both nominative and accusative:

Nominative: hrvatski (language) – Croatian
Accusative: hrvatski – I learn/study Croatian (Učim hrvatski.)

So učim hrvatski is “I’m learning Croatian (language).”

What does kod kuće literally mean, and why is it used for “at home”?

Kod kuće literally means “by/at the house”.

kod = by, at (someone’s place) → takes genitive
kuća = house → genitive singular: kuće

So grammatically:

kod
- kuće (genitive) = kod kuće = at the house → at home

This is a fixed expression meaning “at home”. It’s the most common and natural way to say at home.

You will also hear:

doma – also “at home” (colloquial, very common)
u kući – “in the house” (more literal, physical inside a building)

What case is kuće in, and why?

Kuće is genitive singular of kuća (house).

The preposition kod always requires the genitive:

kod prijatelja – at a friend’s (place)
kod liječnika – at the doctor’s
kod kuće – at home

So kuća → kuće because kod demands genitive.

Is the word order fixed? Can I say Ja učim hrvatski kod kuće, a on radi u gradu?

The word order in Croatian is fairly flexible, especially for pronouns and adverbs, but it affects emphasis.

Your alternative is correct:

Ja učim hrvatski kod kuće, a on radi u gradu.

Differences in feel:

On radi u gradu, a ja učim hrvatski kod kuće.
Slightly emphasizes him first, then contrasts with I.
Ja učim hrvatski kod kuće, a on radi u gradu.
Slightly emphasizes me first, then contrasts with him.

Both are natural; you choose depending on what you want to foreground.

In English we say “He works in the city and I study Croatian at home.” Why isn’t it i ja instead of a ja?

If you use i, it sounds more like you’re piling up similar actions, not contrasting them:

On radi u gradu i ja učim hrvatski kod kuće.
This sounds a bit odd, like two separate statements forced together.

A is much better for:

soft contrast,
parallel but different actions (he does X, I do Y),
comparison of two subjects.

So Croatian prefers:

On radi u gradu, a ja učim hrvatski kod kuće.

This matches the English nuance “He works in the city, while I study Croatian at home.”

Could we say On radi u gradu, a učim hrvatski kod kuće without ja?

You can drop ja grammatically, because učim already shows the subject is I. But in this specific sentence it’s better to include ja:

On radi u gradu, a ja učim hrvatski kod kuće. – Natural, clear contrast.
On radi u gradu, a učim hrvatski kod kuće. – Grammatically OK, but sounds a bit incomplete; the contrast he vs I is weaker.

Because the sentence is built on the contrast of subjects, Croatian usually keeps the pronouns on and ja here.