U laboratoriju rade pokus, mali eksperiment s vodom i svjetlom.

In the laboratory they are doing a trial, a small experiment with water and light.

Breakdown of U laboratoriju rade pokus, mali eksperiment s vodom i svjetlom.

• mali: small
• voda: water
• u: in
• i: and

Questions & Answers about U laboratoriju rade pokus, mali eksperiment s vodom i svjetlom.

Why is it u laboratoriju and not u laboratorija or u laboratoriji?

The base (nominative) form of the noun is laboratorij (masculine).

After the preposition u meaning in (when it indicates location, not movement), Croatian uses the locative case.
The locative singular of laboratorij is laboratoriju.

So:

• Nominative: laboratorij – the laboratory (subject form)
• Locative: (u) laboratoriju – in the laboratory

Forms like laboratorija or laboratoriji would be other cases or wrong for this context.
Here, u laboratoriju literally means in the laboratory in the sense of “inside that place.”

Who is the subject of rade? Why isn’t there a Croatian word for they?

The verb rade is the 3rd person plural present of raditi (to do, to work):
oni rade = they do / they are doing.

In Croatian, the subject pronoun (oni = they) is usually omitted when it’s clear from context and from the verb ending. This is called a “pro‑drop” language: the personal ending on the verb itself tells you the subject.

So the sentence really means:

• (Oni) rade pokus… – They are doing an experiment…

But oni is left out because it’s not needed.