U ogledalu vidim da mi je kosa preduga.

U literally means in. With u ogledalu you’re describing what you see in the mirror’s reflection (inside the “mirror image”), so Croatian normally uses u + locative.

• u ogledalu = in the mirror / in the reflection
• na ogledalu would sound like on the surface of the mirror (e.g., dirt, a sticker, a crack).

Because u (when meaning “in/inside” as a location) requires the locative case.

• Nominative: ogledalo (a mirror)
• Locative singular: (u) ogledalu

So: u ogledalu = “in the mirror.”

Da introduces a content clause (a clause expressing what you realize/notice/claim).

• Vidim da... = “I see that... / I can tell that...”

It’s very common after verbs like vidjeti (to see), znati (to know), misliti (to think), čuti (to hear).

Sometimes, but it changes the nuance:

• vidim da mi je kosa preduga = I can tell/notice that my hair is too long (a conclusion).
• vidim kako... often means “I see how...” (method) or “I watch as...” (an unfolding action).

For a judgment like too long, da is the natural choice.

Mi is the dative clitic meaning to me / for me. It marks the person affected/possessing in a very common Croatian construction:

• Kosa mi je preduga ≈ “My hair is too long” (literally: “Hair is too long to me.”)

It’s often used instead of a possessive adjective like moja.

You can. Both are correct, but they feel different:

• Kosa mi je preduga sounds more natural and conversational, and it highlights my perspective.
• Moja kosa je preduga is also fine, but moja can sound slightly more emphatic/contrastive (e.g., my hair, not yours).

Croatian has a typical clitic order in a clause: short unstressed words (clitics) go early and in a fairly fixed sequence. In this sentence:

• mi (dative clitic) normally comes before
• je (the clitic form of biti, “is”)

So mi je is the standard order.

Yes. Je is the present tense, 3rd person singular of biti = is. In the subordinate clause, the structure is:

• kosa (subject) + je (is) + preduga (too long)

So: (that) my hair is too long.

Because the adjective must agree with the noun kosa:

• kosa is feminine singular
• therefore: preduga (feminine singular)

Other forms would match different nouns:

• masculine singular: predug
• neuter singular: predugo
• feminine plural: preduge, etc.

Here pre- means too / excessively.

• duga = long (feminine)
• preduga = too long

It’s a productive pattern: prevelik (too big), preskup (too expensive), prebrz (too fast).

Croatian usually drops subject pronouns because the verb ending already shows the person:

• vidim = “I see”

You can add ja for emphasis or contrast:

• Ja u ogledalu vidim... = I see (maybe others don’t).

Yes, Croatian word order is flexible, with small shifts in emphasis:

• U ogledalu vidim da mi je kosa preduga. (neutral)
• ...da je moja kosa preduga. (uses moja; slightly more explicit/emphatic)
• ...da mi je preduga kosa. (puts preduga earlier; can feel a bit more expressive)

The key constraints are mostly about clitic placement (e.g., keeping mi je together in the right position).