Learn Hardcore Croatian: U kabini mi je pretoplo, ali veličina jakne je dobra. - It’s too warm for me in the fitting room, but the jacket size is good.

Because u + locative is used for location (where?): U kabini = in the fitting room/cabin.
U + accusative is used for movement (to where?): U kabinu = into the fitting room/cabin (entering it).

What case is kabini, and how do I know?

kabini is locative singular of kabina (feminine noun).
Typical locative ending for many feminine nouns is -i: kabina → (u) kabini.

What does mi je mean here, and why is mi used?

This is an impersonal “feels like” type construction: mi is dative meaning to me / for me, and je is is.
So mi je pretoplo literally means it is too warm to/for me → “I’m too hot / it’s too warm for me (in there).”

Can I say meni je pretoplo instead of mi je pretoplo?

Yes. meni is the stressed/full form, used for emphasis or contrast.

Neutral: U kabini mi je pretoplo.
Emphatic/contrastive: U kabini meni je pretoplo (a tebi nije). = “In the fitting room, I’m too hot (but you aren’t).”

What part of speech is pretoplo? Why does it end in -o?

Here pretoplo functions as a predicative adjective/adverb-like form (often called a “state” expression): (it is) too warm.
The -o is the neuter form used in impersonal statements like:

Hladno je. = “It’s cold.”
Vruće je. = “It’s hot.”
Pretoplo je. = “It’s too warm.”

Is pretoplo the same as prevruće? When would I use each?

Both mean too hot/too warm, but the nuance differs:

pretoplo = “too warm” (often slightly milder or more “warmth” oriented)
prevruće = “too hot” (more “heat” intense)
In everyday speech, both are common and often interchangeable.

Why is the word order U kabini mi je pretoplo and not Mi je pretoplo u kabini?

Croatian word order is flexible, but clitics like mi and je have strong placement rules: they usually come early in the clause (often in “second position”).
Starting with U kabini sets the scene (“in the fitting room”), then the clitic cluster mi je follows naturally: U kabini | mi je | pretoplo.
You can move u kabini later for emphasis, but it may sound more marked: Mi je pretoplo u kabini (less neutral).

Why is je repeated: mi je pretoplo, ali ... je dobra?

Because there are two separate clauses, each with its own predicate:
1) mi je pretoplo = “it’s too warm for me”
2) veliភina jakne je dobra = “the jacket size is good”
In casual speech, the second je is often dropped: ... ali veličina jakne dobra. (still common and natural).

What case is jakne, and why is it not jakna?

jakne is genitive singular of jakna (“jacket”).
It’s used because veličina (“size”) is typically expressed as X of Y → veličina jakne = the jacket’s size / the size of the jacket.

Does veličina jakne mean “the size of the jacket” or “a jacket size (like S/M/L)”?

It can cover both, but in this sentence it most naturally means the jacket’s size / the size (fit) of the jacket is good—i.e., the size you chose fits well.
If you wanted to stress the labeled size, you might add context, e.g. Broj jakne je dobar (“the size number is good”) in some varieties.

Why is it dobra and not dobro?

Because dobra agrees with veličina, which is feminine singular.
Adjective agreement in Croatian matches gender + number + case:

veličina (fem. sg.) → dobra (fem. sg.)
If it were a neuter noun, you’d use dobro, e.g. mjesto je dobro (“the place is good”).

Where are the articles like “the” and “a”? How do I know if it’s “the cabin” or “a cabin”?

Croatian has no articles, so definiteness is inferred from context.
Here, U kabini likely means in the (fitting) cabin you’re currently in, and veličina jakne likely refers to the jacket you’re trying on—but Croatian doesn’t need the/a to express that.