Žena je zaljubljena u prijatelja.

Because žena is feminine singular, and zaljubljen must agree with it.

• zaljubljen = masculine singular
• zaljubljena = feminine singular
• zaljubljeno = neuter singular

So:

• Muškarac je zaljubljen. = The man is in love.
• Žena je zaljubljena. = The woman is in love.

Je is the 3rd person singular present form of biti (to be), so it means is.

The sentence structure is:

• Žena = woman
• je = is
• zaljubljena = in love
• u prijatelja = with / in a friend

So je is necessary because Croatian, like English here, uses to be in this kind of sentence.

Yes, u often means in or into, but with zaljubljen / zaljubljena, Croatian uses the fixed expression:

• zaljubljen u + accusative

This means in love with someone.

So:

• zaljubljena u prijatelja = in love with a friend / the friend

Even though the literal pieces may look like in friend, the whole expression should be learned as an idiom: biti zaljubljen u nekoga = to be in love with someone.

Because after u in this expression, Croatian uses the accusative case.

The basic form is:

• prijatelj = friend

But in the accusative singular, a masculine animate noun changes to:

• prijatelja

That is why you get:

• u prijatelja

This is a very important pattern: for masculine animate nouns, the accusative singular usually looks like the genitive singular.

Examples:

• vidim prijatelja = I see a friend
• poznajem čovjeka = I know the man
• voli psa = he/she loves the dog

Grammatically, prijatelj means male friend. In some contexts it can also mean boyfriend, but not automatically.

In this sentence, without extra context, the safest reading is:

• a male friend or
• the male friend

If Croatian wants to make boyfriend clearer, context usually helps, or a different wording may be used.

Also note:

• prijatelj = male friend
• prijateljica = female friend

So u prijatelja specifically refers to a male person.

Because Croatian does not have articles like English a/an/the.

So žena can mean:

• a woman
• the woman

And prijatelja can mean:

• a friend
• the friend

The exact meaning depends on context.

That is why one Croatian sentence can often be translated in more than one natural English way.

Yes. Žena can mean either woman or wife, depending on context.

So this sentence could theoretically mean:

• The woman is in love with a friend or
• The wife is in love with a friend

If the meaning shown to the learner is woman, then that is the intended reading here. But it is good to know that žena has both uses.

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

The neutral order here is:

• Žena je zaljubljena u prijatelja.

But other orders are possible for emphasis or style, for example:

• Zaljubljena je u prijatelja. = She is in love with a friend.
• U prijatelja je žena zaljubljena. = It is with a friend that the woman is in love.

Even when the order changes, the case endings help show who is doing what.

In this sentence, zaljubljena usually means in love as a state.

So:

• Žena je zaljubljena u prijatelja. = The woman is in love with a friend.

It describes her condition, not necessarily the exact moment she fell in love.

Croatian often uses zaljubljen / zaljubljena as an adjective meaning someone is in love.