Vidim se u ogledalu.

In Croatian, the subject pronoun is often omitted because the verb ending already shows who is doing the action.

• vidim = I see
• the ending -im tells you the subject is I

So Vidim se u ogledalu is completely natural without ja.

You could say Ja se vidim u ogledalu, but that usually adds emphasis, like I can see myself.

Se is a reflexive particle. In this sentence, it shows that the action goes back to the subject:

• vidim = I see
• vidim se = I see myself

So se is what gives the idea of myself here.

It is very common in Croatian with reflexive verbs and reflexive meanings.

Both are possible, but they are not used in exactly the same way.

• vidim se = the normal, neutral way to say I see myself
• vidim sebe = more emphatic, like I see myself / my own self

So in an ordinary statement, se is the most natural choice.

Because the preposition u here means in, and after u with a location meaning, Croatian uses the locative case.

The noun is:

• ogledalo = mirror (dictionary form, nominative)
• u ogledalu = in the mirror (locative singular)

So ogledalu is the correct case form after u when you are talking about being in a place or location.

It is locative singular.

The noun ogledalo is a neuter noun, and its locative singular form is ogledalu.

Here is the pattern:

• nominative: ogledalo
• locative: ogledalu

So u ogledalu literally means in the mirror.

Because u can be used with different cases depending on the meaning.

• u + locative = location, in / inside
• u + accusative = movement into something

Here, there is no movement. The sentence describes where you see yourself: in the mirror. That is location, so Croatian uses locative:

• u ogledalu = in the mirror

Compare:

• Ja sam u sobi. = I am in the room.
• Idem u sobu. = I am going into the room.

Yes. Vidim se is a complete sentence and means I see myself.

The phrase u ogledalu just adds extra information about where you see yourself.

So the structure is:

• Vidim se = complete idea
• u ogledalu = location/context

Yes. Croatian word order is fairly flexible, although some orders sound more natural than others depending on emphasis.

The most neutral order is:

• Vidim se u ogledalu.

Other possible orders include:

• U ogledalu se vidim.
• Se vidim u ogledalu. — possible in some contexts, but less neutral
• Ja se vidim u ogledalu. — adds emphasis to I

Changing the order usually changes the focus or emphasis, not the core meaning.

This is a very useful distinction.

• vidjeti / vidim = to see
• gledati / gledam = to look / watch

So:

• Vidim se u ogledalu. = I see myself in the mirror.
• Gledam se u ogledalu. = I am looking at myself in the mirror.

The first is about perception: you can see your reflection.
The second is about the action of looking.

In many contexts, both can make sense, but they are not identical.

Vidim is present tense, first person singular.

It comes from the verb vidjeti or videti depending on the standard/variant, meaning to see.

So:

• vidim = I see
• se = myself
• u ogledalu = in the mirror

Altogether, it is a present-tense sentence.

You negate it by putting ne before the verb:

• Ne vidim se u ogledalu. = I do not see myself in the mirror.

That is the normal way to make negative sentences in Croatian.

A simple approximate pronunciation for an English speaker is:

• VEE-deem seh oo oh-gleh-DAH-loo

A few helpful points:

• v is like English v
• i is like ee in see
• e is like e in bed, but often a bit cleaner/purer
• u is like oo in food
• lj does not appear here, so that part is simple
• g is always a hard g, as in go

Croatian spelling is quite phonetic, so once you know the sounds, pronunciation is usually straightforward.