Ovaj čaj je prejak za mene.

Ovaj means this, and it has to agree with the noun it describes. Čaj is masculine singular nominative, so the matching form is ovaj.

Compare:

• ovaj čaj = this tea
• ova kava = this coffee
• ovo vino = this wine

Je is the 3rd person singular present of biti = to be. It means is.

So the structure is:

• Ovaj čaj = this tea
• je = is
• prejak = too strong
• za mene = for me

The base adjective is jak = strong. The prefix pre- adds the idea of too, overly, or excessively. So prejak means too strong.

This pattern is very common in Croatian:

• presladak = too sweet
• prehladan = too cold
• prevruć = too hot

Because the adjective agrees with čaj. Since čaj is masculine singular, the adjective is also masculine singular: prejak.

If the noun changed, the adjective would change too:

• Ova kava je prejaka. = This coffee is too strong.
• Ovo vino je prejako. = This wine is too strong.

Za here means for, and mene is the form of I/me used after a preposition here. So za mene means for me.

It expresses that something is too strong relative to your taste, tolerance, or preference.

Ja is the basic form meaning I. After the preposition za, Croatian uses a different pronoun form: mene.

So:

• ja = I
• mene = me

This is a case change. After za, you normally use the accusative form, which here is mene.

Yes. That is also very natural Croatian.

Mi means to me or for me in the dative, and Croatian often uses this structure with adjectives like prejak.

So both are good:

• Ovaj čaj je prejak za mene.
• Ovaj čaj mi je prejak.

The version with za mene sounds a bit more explicit, while mi je is very common and compact.

Yes. Croatian word order is more flexible than English word order.

The sentence Ovaj čaj je prejak za mene is neutral and natural. But you may also hear:

• Ovaj je čaj prejak za mene.
• Prejak je ovaj čaj za mene.

These alternatives change emphasis slightly, not the basic meaning.

No. The adjective prejak agrees with čaj, not with the speaker. Since čaj is masculine, prejak stays masculine no matter who says it.

What would change is only the person in za mene if you mean someone else:

• za tebe = for you
• za njega = for him
• za nas = for us

A rough English-style guide is:

OH-vai chai yeh PREH-yak za MEH-neh

A few useful sound notes:

• j is pronounced like English y
• č is like ch in chocolate
• aj sounds like eye

So čaj sounds very close to English chai.

No. Prejak can be used for many things that are too strong or intense.

For example:

• prejaka kava = coffee that is too strong
• prejak miris = a smell that is too strong
• prejak okus = a flavor that is too strong
• prejako svjetlo = light that is too strong/too bright

So it is a general adjective, not one used only for drinks.