Learn Hardcore Croatian: On trenira svaki dan ujutro.

Croatian is a pro‑drop language: the subject pronoun (I, you, he, she…) is usually omitted because the verb ending already shows who is doing the action.

On trenira svaki dan ujutro. = He trains every day in the morning. (neutral, maybe slight emphasis on he in contrast to someone else)
Trenira svaki dan ujutro. = He trains every day in the morning. (perfectly normal, everyday style)

You typically keep On only for:

emphasis (On trenira, a ne njegov brat. = He trains, not his brother.)
contrast
or when the subject might otherwise be unclear from context.

What exactly does the verb trenirati mean in this sentence?

Here trenirati means to train / to work out / to practice sports. It refers to physical or sports training, not to using a train as transport.

So:

On trenira ≈ He trains / He works out / He practices (sports).

It does not mean he takes the train (for that you’d say something with vlak – train – like On ide vlakom = He goes by train).

Why is the verb form trenira used, and how would I conjugate trenirati?

Trenira is the 3rd person singular present tense of trenirati.

Present tense of trenirati:

ja treniram – I train
ti treniraš – you train (singular, informal)
on/ona/ono trenira – he/she/it trains
mi treniramo – we train
vi trenirate – you train (plural or formal)
oni/one/ona treniraju – they train

So On trenira = He trains / He is training (Croatian uses the same form for both simple and continuous meanings; context decides which is meant).

Can trenira mean both “he trains” and “he is training (right now)”?

Yes. Croatian does not have a separate continuous (‑ing) form like English.

On trenira can mean:

He trains / He works out (regularly). (habitual action, like in your sentence)
He is training / working out (right now). (ongoing action)

Context and sometimes adverbs (like sad = now, svaki dan = every day) tell you whether it’s habitual or happening at the moment.

Why is it svaki dan here and not svakog dana? Is there a difference?

Both svaki dan and svakog dana are correct, and both mean every day.

svaki dan – more neutral, very common in everyday speech.
svakog dana – slightly more formal/literary or emphatic in some contexts, but also very common.

In your sentence, you could say:

On trenira svaki dan ujutro.
On trenira svakog dana ujutro.

Both are natural. For learning, you can safely use svaki dan as your default.

What grammatical form is svaki in, and why?

In svaki dan, the word svaki is:

masculine
singular
accusative case

It agrees with dan (day), which is:

masculine
singular
used as a time expression in accusative (= duration / frequency).

So:

svaki dan = every day (masc. sg. acc.)
If it were feminine, you’d have svaku večer (every evening; večer is feminine).
Neuter example: svako jutro (every morning; jutro is neuter).

What is ujutro exactly? Is it one word or “u jutro”? How is it used?

Ujutro is normally written as one word and works as an adverb of time, meaning in the morning.

On trenira ujutro. = He trains in the morning.

Compare:

ujutro (one word) – adverb “in the morning”
u jutro (two words) – literally “into the morning / in the morning (of a specific day)”, used when jutro is clearly a noun in a larger phrase, e.g.
U jutro tog dana bilo je hladno. = On the morning of that day, it was cold.

For simple time expressions like yours, use ujutro as one word.

Could the word order be different, like On ujutro trenira svaki dan or Trenira ujutro svaki dan?

Yes, Croatian word order is quite flexible. These are all grammatically correct:

On trenira svaki dan ujutro.
On trenira ujutro svaki dan.
On ujutro trenira svaki dan.
Trenira svaki dan ujutro.
Svaki dan ujutro trenira.

The main difference is emphasis / focus, not basic meaning. For example:

Starting with Svaki dan ujutro puts more focus on when he trains.
Keeping On at the start, On trenira…, can highlight he in contrast with others.

In everyday speech, something like On trenira svako jutro or On ujutro trenira svaki dan would sound very natural.

Is On trenira svaki dan ujutro a natural sentence for native speakers, or would they usually say it differently?

It’s grammatically correct and understandable, but many native speakers might simplify it slightly.

More common, very natural options would be:

On trenira svako jutro. – He trains every morning.
On trenira svaki dan. – He trains every day.
On trenira svaki dan ujutro. – fine as-is, just a bit longer; literally every day in the morning.

So your sentence is okay, but if you just mean “every morning”, On trenira svako jutro (or svakog jutra) sounds a bit more compact and typical.

How would I say “He trains every morning” or “He trains every day in the evening” in Croatian?

For He trains every morning:

On trenira svako jutro.
On trenira svakog jutra. (equally correct)

For He trains every day in the evening:

On trenira svaki dan navečer.
or On trenira svaku večer. = He trains every evening.

Patterns to notice:

svako jutro / svakog jutra – every morning
svaku večer / svake večeri – every evening
navečer – in the evening (adverb, like ujutro).

Could I use another verb instead of trenirati, like vježbati? Would the meaning change?

Yes, you can often use vježbati instead of trenirati, but there is a slight nuance:

trenirati – to train, usually in a sports or workout context; can sound a bit more “sporty” or systematic.
vježbati – to exercise / to practice; can be physical exercise, but also practicing music, language, etc.

In your sentence:

On trenira svaki dan ujutro. – He trains / works out every morning.
On vježba svaki dan ujutro. – He exercises every morning (could be sports, gym, or more general physical exercise).

Both are fine for the idea of working out; trenirati leans a bit more toward sports training.