Learn Hardcore Croatian: Eksperiment je jednostavan, ali im pomaže otkriti zašto se voda brzo zagrije. - The experiment is simple, but it helps them discover why the water heats up quickly.

In Croatian, the verb biti (to be) in short forms like je, sam, si is a clitic and usually goes in the second position in the clause, not necessarily right before or after the subject in a rigid way.

Here, the first “slot” in the clause is Eksperiment, so je naturally comes right after it:

Eksperiment je jednostavan = The experiment is simple.

You normally would not say Je eksperiment jednostavan in a neutral statement; that order would sound like a yes/no question or just wrong.

Why is it jednostavan and not jednostavno?

Jednostavan is an adjective and must agree with the noun eksperiment:

eksperiment is masculine, singular, nominative.
So the adjective in predicate position is jednostavan (masc. sg.).

Jednostavno is the adverb (“simply”) or the neuter singular adjective form.
Here we’re saying “The experiment is simple”, so we need an adjective that matches eksperiment:

Eksperiment je jednostavan. ✅ (The experiment is simple.)
Eksperiment je jednostavno. ❌ (This sounds wrong; “it is simply” what?)

What does im mean, and why is that form used?

Im is a short (clitic) dative plural pronoun meaning “to them”.

It’s the dative of oni (they):

oni → they
njima → to them (full form)
im → to them (short/clitic form)

In ali im pomaže…, the verb pomaže (“helps”) takes an indirect object in the dative:

pomaže komu? čemu? → im (“helps them / helps to them”)

Because im is a clitic, it also tends to appear early in the clause, usually in the second position:

ali im pomaže… (but it helps them…)
You would not normally say ali pomaže im in neutral speech; that sounds marked or poetic.

Why is it pomaže otkriti and not something like pomaže da otkriju?

Both structures are possible, but they are slightly different:

pomaže otkriti (helps to discover)
- Verb pomaže
  - infinitive otkriti.
- Very direct and compact construction, common in Croatian.
pomaže da otkriju (helps [them] so that they discover)
- Verb pomaže
  - da-clause with a finite verb otkriju (3rd person plural).
- Also correct, a bit more explicit, sometimes a bit more formal or explanatory.

In your sentence, pomaže otkriti is perfectly natural and sounds smooth:

ali im pomaže otkriti zašto…
“but it helps them discover why…”

The “to” in English is just how English marks the infinitive; Croatian uses the bare infinitive otkriti here, no extra word needed.

What is the difference between otkriti and otkrivati?

This is a question of aspect:

otkriti – perfective: to discover (as a completed event, reaching a result).
otkrivati – imperfective: to be discovering / to discover repeatedly / ongoing process.

In pomaže otkriti, the idea is “helps (them) to reach the discovery (once, as a result).”
If you used otkrivati, it would emphasize an ongoing process of discovering things, not one particular explanation.

What does se do in zašto se voda brzo zagrije?

Se is a reflexive clitic. Here it is part of the reflexive verb zagrijati se:

zagrijati – to heat (something)
zagrijati se – to heat up, to get warm

So zašto se voda brzo zagrije literally means “why the water quickly heats itself up / becomes warm quickly.”

Without se, zagrijati would usually need an object:

zagrijati vodu – to heat the water
voda se zagrije – the water heats up (it gets heated / becomes warm)

Why is the word order zašto se voda brzo zagrije and not zašto voda se brzo zagrije?

Again, this is because se is a clitic. In a clause like this, clitics tend to appear in the second position:

1st element: zašto
2nd position: se
then the rest: voda brzo zagrije

So:

zašto se voda brzo zagrije ✅ (natural word order)
zašto voda se brzo zagrije ❌ (sounds wrong / very unnatural)

Clitics like se, je, sam, si, im, mu, ga are very restricted in where they can go, and “second position” is the basic rule in simple clauses.

Why is brzo used instead of brza?

Brzo is an adverb (“quickly, fast”).
Brza is an adjective (“quick, fast” feminine singular).

In voda brzo zagrije, brzo modifies the verb zagrije (how does it heat up? Quickly.), so an adverb is needed:

voda brzo zagrije – the water heats up quickly.

If you said voda je brza, you’d be using brza as an adjective describing the noun (e.g. in some metaphorical context “the water is fast”), but that’s not what is happening here. We want to describe the manner of heating up, so we use the adverb brzo.

Why is it zagrije and not zagrijava?

This is again about aspect and nuance:

zagrijati se → zagrije se (perfective: to heat up, reach the warm state)
zagrijavati se → zagrijava se (imperfective: to be heating up, process, repeated/habitual)

In the sentence, they want the idea of the water gets heated up (reaches that state), which matches the perfective verb zagrijati se, present tense zagrije.

Croatian often uses present tense of a perfective verb to talk about:

future-like result: what will eventually happen,
general truths about how things reach a result.

So zašto se voda brzo zagrije is like saying “why the water (ends up) heating up quickly”.
If you said zašto se voda brzo zagrijava, you’d highlight the ongoing process of heating, not so much the end result.

What tense is zagrije, and why does it seem to have a future meaning?

Formally, zagrije is the present tense of the perfective verb zagrijati se.

In Croatian (and other Slavic languages), the present tense of perfective verbs often has a future or result-oriented meaning, for example:

Kad klikneš, program se otvori. – When you click, the program opens.
Ako dodaš sol, voda brže zavrije. – If you add salt, the water boils faster.

So zašto se voda brzo zagrije is “why the water (comes to) heat up quickly”, with a general, almost future-like flavor. English usually uses simple present or “gets heated” for that kind of general explanation, but Croatian uses the perfective present.