Learn Hardcore Croatian: On trči sporije kad je umoran. - He runs more slowly when he is tired.

In Croatian, the subject pronoun (on, ona, oni, etc.) is often dropped, because the verb ending already shows the person.
So both are correct:

On trči sporije kad je umoran.
Trči sporije kad je umoran.

The version with on is a bit more emphatic, or useful if you want to contrast him with someone else (e.g. On trči sporije, ali ona trči brže.).

What exactly is trči (person, tense, infinitive)?

Trči is:

the 3rd person singular form (he/she/it)
of the verb trčati (to run)
in the present tense

Full present conjugation of trčati:
ja trčim, ti trčiš, on/ona/ono trči, mi trčimo, vi trčite, oni/one/ona trče.

Why is it sporije and not sporo or spor?

sporo = “slowly” (basic adverb)
sporije = “more slowly / slower” (comparative adverb)
spor = “slow” (adjective, masculine form)

In On trči sporije, sporije modifies the verb trči, so it must be an adverb (how does he run? more slowly), not an adjective.
So On trči sporije = “He runs more slowly / slower.”

How is sporije formed? How would I say “the slowest”?

From the adjective spor:

adjective: spor (m), spora (f), sporo (n)
comparative adjective: sporiji, sporija, sporije
superlative adjective: najsporiji, najsporija, najsporije

The adverb forms (how?) use the neuter form:

sporo – slowly
sporije – more slowly / slower
najsporije – the most slowly / slowest

So: On trči najsporije. = “He runs the slowest / the most slowly.”

What does kad mean exactly? Is there any difference between kad and kada?

kad means when.
kada is a slightly longer, more formal or emphatic version. In everyday speech they are usually interchangeable.

So you can say:

On trči sporije kad je umoran.
On trči sporije kada je umoran.

The meaning is the same; kad is just the shorter, more colloquial form.

Why is it kad je umoran, not kad je umoranom or something else? Which case is umoran?

Umoran is an adjective meaning tired, and here it is in the nominative masculine singular, agreeing with the (understood) subject on.

In Croatian, after biti (to be), the adjective normally stands in the nominative, not another case:

On je umoran. – He is tired.
Ona je umorna. – She is tired.

So kad je umoran literally mirrors English “when he is tired.”

Can I change the word order, for example: Kad je umoran, on trči sporije? Does it sound natural?

Yes, that is completely correct and natural:

Kad je umoran, on trči sporije.
On trči sporije kad je umoran.

The meaning is the same. Putting kad je umoran at the beginning emphasizes the condition (“When he’s tired…”), while keeping it at the end feels a bit more neutral. Both are very common.

Why is je in the middle of kad je umoran? Can I say kad umoran je?

You cannot say kad umoran je; that sounds wrong in Croatian.
Je is the unstressed form of biti (to be), and Croatian has a rule that such short unstressed words (clitics) usually stand in second position in the clause.

In the clause kad je umoran:

kad is the first word
je comes immediately after it (second position)
umoran comes last

So kad je umoran follows normal Croatian word order for clitics.

Do I need a comma before kad: On trči sporije, kad je umoran?

In this sentence, no comma is required, and the most common written form is:

On trči sporije kad je umoran.

When the kad-clause comes first, you normally use a comma:

Kad je umoran, on trči sporije.

Putting a comma in On trči sporije, kad je umoran is usually not necessary and can look a bit unusual in such a short sentence.

Can this Croatian present tense talk about a general habit, like the English “When he is tired, he runs slower”?

Yes. Croatian present tense (trči, je umoran) is very often used for general truths and habits, just like in English.

So:

On trči sporije kad je umoran.
means “Whenever he is tired, he runs more slowly,” describing a general, repeated behavior, not just one specific occasion.

How would I say “He will run slower when he is tired” (future meaning)?

For a clearer future meaning you normally change the main verb (and often the “when”-clause too):

On će trčati sporije kad bude umoran.
= He will run slower when he is (will be) tired.

You could also hear:

On će trčati sporije kad bude jako umoran. – when he gets really tired.

In everyday speech, context sometimes makes it clear even with the present, but će + infinitive (će trčati) is the standard future.

How would this sentence change if we talk about a woman or about more people?

For a woman:

Ona trči sporije kad je umorna.

Changes:

subject pronoun: on → ona
adjective: umoran → umorna (feminine singular nominative)

For several men / mixed group:

Oni trče sporije kad su umorni.

Changes:

verb: trči → trče (3rd person plural)
verb “to be”: je → su
adjective: umoran → umorni (masculine plural nominative)

Is there any difference between using sporije and polako here?

Both can be used, but the nuance is slightly different:

On trči sporije kad je umoran. – He runs slower / more slowly (compared to his normal speed).
On trči polako kad je umoran. – He runs slowly (describes the manner more absolutely, without explicit comparison).

To keep the comparative idea “slower than usual,” sporije is the most direct translation.