Mučno mi je u autobusu.

Croatian often uses an impersonal structure for physical feelings and states.

So mučno mi je is literally something like:

• mučno = nauseous / sickening
• mi = to me
• je = is

So the structure is closer to It is nauseous to me than to I am nauseous.

This is very natural in Croatian for things like:

• hladno mi je = I’m cold
• vruće mi je = I’m hot
• dosadno mi je = I’m bored
• mučno mi je = I feel nauseous / sick

English usually uses I am / I feel, but Croatian often uses this impersonal pattern instead.

Here mučno means something like:

• nauseous
• sick
• queasy
• feeling unwell

It can also more generally mean unpleasant or distressing in other contexts, so its meaning depends on the sentence.

In Mučno mi je u autobusu, it most naturally refers to feeling sick or nauseous, especially because being in a bus often suggests motion sickness.

Yes — mi is the dative form meaning to me.

In this kind of sentence, Croatian marks the person experiencing the feeling with the dative:

• mučno mi je = I feel nauseous
• mučno ti je = you feel nauseous
• mučno mu je = he feels nauseous

So even though English says I feel sick, Croatian says it more like it is sickening to me.

That is why mi appears instead of a nominative pronoun like ja.

Because the verb agrees with the predicate mučno, not with I.

Here mučno is a neuter singular predicate adjective/adverbial form, so Croatian uses je = is.

Compare:

• Ja sam umoran. = I am tired.
  Here umoran agrees with ja (masculine singular), so you use sam.

But:

• Mučno mi je. = I feel nauseous.
  Here the structure is impersonal, so you use je, not sam.

So this is not I am X grammatically. It is an impersonal expression.

After u meaning in / inside / at for a location, Croatian uses the locative case.

So:

• dictionary form: autobus
• locative singular: autobusu

That gives:

• u autobusu = in the bus / on the bus

So autobusu is in the locative singular.

Grammatically, u autobusu literally means in the bus, but in English the natural translation is often on the bus.

Croatian commonly uses u with buses, cars, trains, etc. when talking about being inside them.

So in practice:

• u autobusu = on the bus / in the bus

depending on what sounds natural in English.

Yes. Croatian word order is flexible, and the sentence can be rearranged for emphasis.

For example:

• Mučno mi je u autobusu.
• U autobusu mi je mučno.

Both are natural.

The difference is mainly emphasis:

• Mučno mi je u autobusu focuses first on the feeling.
• U autobusu mi je mučno focuses first on the location: On the bus, I feel sick.

One thing to remember: mi and je are clitics, so they usually appear near the beginning of the clause, often in second position.

Yes, but it changes the tone.

• mi = normal, unstressed form
• meni = stressed/emphatic form

So:

• Mučno mi je u autobusu. = neutral, everyday
• Meni je mučno u autobusu. = emphasizes me, as in I’m the one who feels sick on the bus

You would usually use meni for contrast or emphasis, not as the default choice.

Yes, it is natural.

That said, depending on the speaker and region, you may also hear:

• Zlo mi je u autobusu. = I feel sick on the bus.
• Muka mi je u autobusu. = I feel nauseous / sick on the bus.

These are also common.

A useful difference:

• Mučno mi je = I feel nauseous / queasy
• Pozli mi u autobusu = I get sick / I start feeling sick on the bus

So your sentence is correct and natural, especially for describing the state itself.

Not if you mean I feel nauseous on the bus.

That would sound wrong or very unnatural for this meaning.

The adjective mučan / mučna / mučno can describe something as:

• distressing
• painful
• grueling
• sickening

But when talking about a person’s physical feeling of nausea, Croatian normally uses the impersonal expression:

• Mučno mi je.

So for this meaning, use mučno mi je, not mučan sam.

You negate je:

• Nije mi mučno u autobusu. = I don’t feel sick on the bus.

You can also move things around:

• U autobusu mi nije mučno.

Both are fine; the difference is just emphasis.

Approximately:

• muč- sounds like mooch without the long English vowel
• -no is a short no

So roughly: MOOCH-no

More specifically:

• č is like ch in church
• u is like oo in food, but usually shorter and cleaner
• o is a pure vowel, not a diphthong

So mučno is pronounced about MOOCH-no.