Βρήκαν την ουσία στο νερό.

Βρήκαν is the 3rd person plural aorist active form of βρίσκω (to find).

So it means they found.

A few important points:

• aorist = a simple completed past action
• 3rd person plural = they
• Greek often does not need an explicit subject pronoun, because the verb ending already shows who did the action

So Βρήκαν by itself already tells you they found.

Greek very often omits subject pronouns when they are not needed.

In English, you must say they found.
In Greek, the verb form βρήκαν already includes the idea of they, so adding αυτοί (they) is usually unnecessary unless you want emphasis or contrast.

So:

• Βρήκαν την ουσία στο νερό. = They found the substance in the water.
• Αυτοί βρήκαν την ουσία στο νερό. would sound more like They were the ones who found the substance in the water.

Because την ουσία is the direct object of the verb.

• η ουσία = nominative, used for the subject
• την ουσία = accusative, used for the direct object

Here, the substance is what was found, so it must be in the accusative.

Compare:

• Η ουσία είναι στο νερό. = The substance is in the water.
  (η ουσία = subject)

• Βρήκαν την ουσία στο νερό. = They found the substance in the water.
  (την ουσία = object)

Yes. ουσία is feminine.

That is why its article is feminine too:

• η ουσία = nominative singular
• την ουσία = accusative singular

So the article changes to match the noun’s gender, number, and case.

στο is a very common contraction of:

• σε = in, at, to, on (depending on context)
• το = the (neuter accusative singular here)

So:

• σε το → στο

In this sentence, στο νερό means in the water.

This kind of contraction is standard in modern Greek:

• στο = σε + το
• στη = σε + τη(ν)
• στον = σε + τον
• στους = σε + τους
• etc.

Because νερό is a neuter noun, and in the singular its nominative and accusative forms are the same.

So:

• το νερό = nominative or accusative singular
• after σε, Greek normally uses the accusative
• that gives σε το νερό → στο νερό

Even though the noun itself does not visibly change, the phrase is still grammatically correct because the article and structure show what is happening.

In modern Greek, σε normally takes the accusative.

So in this sentence:

• στο νερό = σε + το νερό
• νερό is functioning as accusative

This is useful to remember because older Greek had different patterns, but in modern Greek, σε + accusative is the normal rule.

Greek word order is more flexible than English word order.

English usually prefers:

• They found the substance in the water.

Greek can say:

• Βρήκαν την ουσία στο νερό.
• Την ουσία βρήκαν στο νερό.
• Στο νερό βρήκαν την ουσία.

These all have roughly the same basic meaning, but the emphasis changes.

In your sentence, starting with Βρήκαν is completely natural. It gives a straightforward narrative feel: Found they... / They found...

Yes. You could say:

• Αυτοί βρήκαν την ουσία στο νερό.

But this usually adds emphasis, contrast, or clarity.

For example, it might imply:

• They found it, not someone else
• or you are explicitly identifying who they are

Without special emphasis, Greek usually prefers the shorter version:

• Βρήκαν την ουσία στο νερό.

Yes, it is somewhat irregular and is worth memorizing.

The verb is:

• βρίσκω = I find

Its aorist stem is different:

• βρήκα = I found
• βρήκες = you found
• βρήκε = he/she/it found
• βρήκαμε = we found
• βρήκατε = you found
• βρήκαν = they found

So you should learn βρίσκω together with its aorist βρήκα.

A simple pronunciation guide is:

VREE-kan tin oo-SEE-a sto ne-RO

A few notes:

• Βρ in Βρήκαν starts with a v sound followed immediately by r
• η in Βρήκαν sounds like ee
• την is often pronounced roughly like tin
• ου sounds like oo
• the stress is on:
  • ΒΡΗ-καν
  • ου-ΣΙ-α
  • νε-ΡΟ

So the stressed syllables are:

• Βρήκαν
• ουσία
• νερό

Not always. ουσία can also mean things like:

• essence
• substance
• main point
• core meaning

The exact sense depends on context.

So although the meaning is already given in your example, it is good to know that ουσία is a flexible word and is not limited to chemistry or physical material.

Yes. In spoken and informal Greek, you may also hear βρήκανε.

Both mean they found.

• βρήκαν = standard and very common
• βρήκανε = also common, often more colloquial or conversational

So if you encounter both, do not worry: they are the same verb form in meaning.