Learn Hardcore Croatian: 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.

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.

Why is it pokus and then mali eksperiment? Aren’t they the same thing?

Yes, pokus and eksperiment both mean roughly experiment, but they’re used slightly differently:

pokus – a bit more native/Slavic word; also tends to sound slightly more practical, concrete, often used in school science, simple demonstrations, tests.
eksperiment – a direct borrowing from international vocabulary (like English experiment). It can sound a bit more formal/scientific, but it’s very common.

In this sentence:

pokus, mali eksperiment s vodom i svjetlom

the comma shows apposition: an experiment, a small experiment with water and light.
So mali eksperiment s vodom i svjetlom is simply explaining/clarifying what kind of pokus it is.

You could also say just:

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

Both would be correct, just with slightly different style/emphasis.

Why is there a comma before mali eksperiment?

The comma is used because mali eksperiment s vodom i svjetlom is in apposition to pokus – it renames or explains it:

pokus, mali eksperiment s vodom i svjetlom
= an experiment, a small experiment with water and light

It’s similar to English:
They are doing an experiment, a small experiment with water and light.

If mali eksperiment were just a regular adjective‑noun phrase tightly attached to pokus, there would be no comma:

rade mali pokus s vodom i svjetlom – they are doing a small experiment with water and light (one phrase, no apposition).

Here the comma adds a slight pause and explanatory tone.

Why is pokus in that form? Isn’t the object supposed to be in the accusative?

Yes, pokus here is in the accusative singular, because it’s the direct object of rade (they are doing an experiment).

For many inanimate masculine nouns ending in a consonant, the accusative singular looks the same as the nominative:

Nominative: pokus – an experiment (as subject)
Accusative: pokus – an experiment (as object)

So the form doesn’t change, but the function in the sentence (subject vs. object) is different.
You recognize the case from the role in the sentence and from the verb, not from a different ending in this particular noun.

Why is it mali eksperiment, not malo eksperiment?

Mali is an adjective, and malo is usually an adverb (or an indeclinable quantifier meaning a little).

mali – small, little as an adjective; it must agree in gender, number, and case with the noun.
eksperiment is masculine singular (here in the accusative, but its form accusative = nominative), so the adjective must be masculine singular too: mali.

So:

mali eksperiment – a small experiment (correct)
malo eksperiment – wrong, because malo doesn’t agree as an adjective with eksperiment.

What case are vodom and svjetlom, and why?

Both vodom and svjetlom are in the instrumental singular.

The preposition s (or sa) in the meaning with (together with, using) requires the instrumental case.

Base forms:

voda (feminine) – nominative singular
→ vodom – instrumental singular (with water)
svjetlo (neuter) – nominative singular
→ svjetlom – instrumental singular (with light)

So s vodom i svjetlom literally means with water and (with) light, and both nouns must be in the same case, the instrumental, because they’re both governed by s.

Why is it s vodom, not sa vodom? When do you use s and sa?

Both s and sa are forms of the same preposition meaning with.

Normally, you use s, but Croatian sometimes adds an extra “a” (making sa) for easier pronunciation, especially:

before words starting with s, z, š, ž or some consonant clusters, to avoid difficult sequences like s svima, s ženom, etc.
or simply by style preference.

Here, s vodom is easy to pronounce and perfectly standard. Sa vodom is also possible, but less common and not needed for pronunciation.

So:

s vodom i svjetlom – neutral, standard
sa svima, sa ženom – often preferred to avoid tongue‑twisters.

Why is it svjetlom and not svjetlost?

Croatian has both:

svjetlo (neuter noun) – light as a source or beam of light, like a lamp, illumination, etc.
- Instrumental: svjetlom – with light
svjetlost (feminine noun) – light more as an abstract phenomenon, brightness, luminosity.
- Instrumental: svjetlošću – with (the) brightness/light

In everyday language, for typical science‑experiment contexts svjetlo is more common:
eksperiment s vodom i svjetlom = an experiment with water and light (e.g. shining light, reflections).

You could say s vodom i svjetlošću, but it would sound more literary/abstract.

Why do we say u laboratoriju and not na laboratoriju?

The preposition:

u = in, inside
na = on, on top of, at (some types of places/events)

A laboratory is a closed space you are inside, so Croatian uses u:

u laboratoriju – in the laboratory

Na laboratoriju would mean literally on the laboratory (e.g., on its roof), which is not what is meant here.

Can the word order change? For example, can I say Rade pokus u laboratoriju, mali eksperiment s vodom i svjetlom?

Yes, Croatian word order is relatively flexible, and your example is grammatically correct:

Rade pokus u laboratoriju, mali eksperiment s vodom i svjetlom.

The basic information is the same, but word order can change emphasis:

U laboratoriju rade pokus… – focuses first on the location (in the laboratory).
Rade pokus u laboratoriju… – starts with what they are doing, then adds where.

All of these are acceptable; the choice depends mainly on what you want to highlight or on stylistic preference.

Why doesn’t Croatian use words like a or the in this sentence?

Croatian (like most Slavic languages) does not have articles like English a/an and the.

So:

U laboratoriju rade pokus… can mean:
- They are doing an experiment in the laboratory.
- They are doing the experiment in the laboratory.

Context tells the listener whether it’s something new/unspecified (an experiment) or known/specific (the experiment). Croatian does this without a separate little word; it relies on context and sometimes on word order and emphasis.

How do you pronounce svjetlom? The consonant cluster looks difficult.

Svjetlom is pronounced approximately:

svje‑tlom

Breakdown:

svj is like sv‑ye together (s + v + palatal j)
tl is just t
- l, both clearly pronounced
The o is a short “o” as in not (British‑like)
Final m is pronounced normally (not nasalized like in some languages)

So a rough English approximation: SVYEHT‑lom, with stress typically on the first syllable: SVJE‑tlom.

What exactly is the implied full sentence in Croatian, with all parts filled in?

A very explicit, fully spelled‑out version could be:

Oni u laboratoriju rade pokus, to jest mali eksperiment s vodom i svjetlom.

Breaking it down:

Oni – they (usually dropped)
u laboratoriju – in the laboratory (locative)
rade – are doing (3rd person plural)
pokus – an experiment (accusative object)
mali eksperiment – a small experiment (in apposition to pokus)
s vodom i svjetlom – with water and light (instrumental after s)

The original sentence just omits oni and to jest (“that is”), which is completely natural in Croatian.