Learn Hardcore Croatian: On stalno radi i zato je umoran. - He works all the time and so he is tired.

On stalno radi. → He works a lot / constantly, maybe with a nuance that it’s too much or annoying.
On uvijek radi. → He always works (in every situation, he is the one who works).

They often overlap in meaning, but stalno can sound more like “non‑stop, continually,” often with a slightly negative or complaining tone.

Can I change the word order and say On radi stalno instead of On stalno radi?

Yes, both are grammatically correct:

On stalno radi. (more common, neutral)
On radi stalno. (also possible; can add a bit more emphasis to stalno)

In Croatian, adverbs like stalno usually come before the verb, but putting them after the verb is also fine and is often used for emphasis or rhythm.

What form of the verb is radi, and what is its infinitive?

The infinitive is raditi (to work).
Radi is:

3rd person singular
present tense
imperfective aspect

So on radi means both he works and he is working. Croatian does not have a separate continuous tense like English; the same present form covers both.

Why is it zato and not zato što or jer? What is the difference?

zato = for that reason / that’s why (an adverb of consequence)
zato što = because (subordinating conjunction)
jer = because (subordinating conjunction, very common)

Your sentence uses zato to introduce the result:

On stalno radi i zato je umoran.
→ He works all the time, and for that reason he is tired.

If you want to introduce the cause clause directly, you use jer or zato što:

On je umoran jer stalno radi.
On je umoran zato što stalno radi.

Meaning is practically the same; zato što can sound a bit more formal or emphatic than jer.

Why do we have both i and zato: i zato? Could I just say zato je umoran?

Yes, you could say:

On stalno radi, zato je umoran.

Here zato still means therefore / that’s why. The i in i zato is just the normal and, linking two clauses:

On stalno radi i zato je umoran.
→ He works all the time, and that’s why he’s tired.

Using i zato sounds very natural and smooth in speech. Leaving out i is also correct, but you usually separate the clauses with a comma in writing.

Can I move je and say On stalno radi i zato umoran je?

Normally, no. That sounds odd or very poetic.

Je is a clitic form of the verb biti (to be) and in standard Croatian clitics want to be in the second position in the clause. In:

On stalno radi i zato je umoran.

the new clause after i basically starts with zato, so je naturally comes right after it (zato je).
Forms like zato umoran je break the usual clitic rule and are only used for special emphasis or poetic effect, not in normal speech.

Do I really need the subject pronoun On, or can I just say Stalno radi i zato je umoran?

You can drop On. Croatian is a pro‑drop language: the verb ending already shows the subject.

On stalno radi i zato je umoran.
Stalno radi i zato je umoran.

Both are correct.
You keep On if you want to:

make it clear who you are talking about
contrast him with someone else (e.g. On radi, a ona se odmara.)

Why is it umoran and not some other form? How does umoran change with gender and number?

Umoran is an adjective meaning tired. In this sentence it is:

masculine
singular
nominative

It agrees with on (he, masculine). Other forms:

ona je umorna (she is tired – feminine singular)
ono je umorno (it is tired – neuter singular)
oni su umorni (they are tired – masc. plural or mixed group)
one su umorne (they are tired – feminine plural)
ona su umorna (they are tired – neuter plural, things/animals, etc.)

Could I say On je stalno umoran instead? Does it mean the same thing?

On je stalno umoran. = He is always/constantly tired.

This focuses on the state (he is constantly tired), not on the explicit cause.

Your original sentence:

On stalno radi i zato je umoran.
→ He works all the time, and that’s why he is tired.

Here the focus is on the cause: he works too much, and that leads to tiredness. So the meaning is related but not identical.

How would I say this in the past, like He worked all day and that’s why he was tired?

You need the past tense (perfect) of raditi and biti:

On je cijeli dan radio i zato je bio umoran.
→ He worked all day and that’s why he was tired.

Structure:

je radio = he worked (auxiliary je
- past participle radio)
je bio = he was (auxiliary je
- past participle bio)

Can I use jer instead of i zato and keep roughly the same meaning?

Yes. A very natural alternative is:

On je umoran jer stalno radi.

Comparison:

On stalno radi i zato je umoran.
→ First says what he does, then explicitly states the result: he is tired for that reason.
On je umoran jer stalno radi.
→ Starts with the result (he is tired) and then gives the cause with because.

Both are common; the choice is mostly about which part you want to present first or emphasize.

Are there other common ways to say stalno radi in Croatian?

Yes, some common synonyms or similar expressions are:

cijelo vrijeme radi – he works the whole time
non‑stop radi – he works non‑stop (informal, uses an English loan)
neprestano radi – he works continuously / without stopping
stalno je na poslu – he is always at work

All keep the idea of him working very often or too much.