Îmi place mirosul de pâine caldă.

Because a plăcea works differently from English to like.

In Romanian, the thing that is liked is the grammatical subject, and the person who likes it is marked indirectly:

• Îmi place mirosul... = The smell ... is pleasing to me
• literally: To me pleases the smell...

So:

• îmi = to me
• place = pleases / is pleasing
• mirosul = the smell

That is why eu is not used here. Eu means I, but Romanian does not structure this idea as I like X. It structures it more like X pleases me.

Îmi is the unstressed indirect object pronoun meaning to me.

It comes from mie = to me, but in normal sentences Romanian usually uses the short clitic form:

• îmi place = I like / it pleases me
• îți place = you like / it pleases you
• îi place = he/she likes / it pleases him/her

You can also say:

• Mie îmi place mirosul de pâine caldă.

That adds emphasis, like As for me, I like the smell of warm bread.

Because the subject is mirosul (the smell), which is singular.

With a plăcea, the verb agrees with the thing that is liked, not with the person who likes it.

So:

• Îmi place mirosul. = I like the smell.
• Îmi plac mirosurile. = I like the smells.

Compare:

• place = singular
• plac = plural

In your sentence, mirosul is one smell, so place is singular.

Mirosul means the smell. The ending -ul is the Romanian definite article attached to the noun.

So:

• miros = a smell / smell
• mirosul = the smell

Romanian usually puts the definite article at the end of the noun instead of using a separate word like the.

Examples:

• băiat = boy
• băiatul = the boy
• miros = smell
• mirosul = the smell

So Îmi place mirosul de pâine caldă is specifically I like the smell of warm bread.

Here de links smell and bread, giving the meaning smell of bread.

This is a very common Romanian pattern:

• o cană de ceai = a cup of tea
• gust de vanilie = taste of vanilla
• miros de cafea = smell of coffee

So:

• mirosul de pâine = the smell of bread

This is the natural way to express what kind of smell it is.

Yes, but the nuance can be a little different.

• mirosul de pâine often means the smell of bread in a general, descriptive sense
• mirosul pâinii uses the genitive and can feel more like the bread’s smell or the smell of the bread

In many contexts both are possible, but mirosul de pâine is very natural for a general sensory description like this sentence.

Because in Romanian, adjectives usually come after the noun.

So:

• pâine caldă = warm bread
• casă mare = big house
• fată frumoasă = beautiful girl

Romanian can sometimes put the adjective before the noun, but that is less neutral and often more literary, emphatic, or stylistically marked. In everyday speech, pâine caldă is the normal order.

Because it agrees with pâine.

• pâine is feminine singular
• so the adjective must also be feminine singular
• therefore: caldă

Compare:

• pâine caldă = feminine singular
• vin cald = masculine/neuter singular
• plăcinte calde = plural

Romanian adjectives change form to match the noun’s gender and number.

It describes pâine.

So the phrase means:

• the smell of warm bread

not:

• the warm smell of bread

You can tell because of agreement:

• pâine is feminine singular, so caldă is feminine singular too
• mirosul is masculine/neuter singular, so if warm described smell, it would be cald

Compare:

• mirosul de pâine caldă = the smell of warm bread
• mirosul cald de pâine = the warm smell of bread (less likely in this context, but grammatically possible)

Because îmi is not the subject. It is the indirect object.

The verb agrees with the subject, which is mirosul.

So in this sentence:

• îmi = to me
• mirosul = subject
• place = singular, matching mirosul

This is one of the most important things to remember with a plăcea: the verb agrees with the thing liked, not with the person who likes it.

Normally, no. Îmi is necessary because it tells you to whom the thing is pleasing.

• Îmi place... = I like...
• Îți place... = you like...
• Îi place... = he/she likes...

Without the pronoun, the sentence would sound incomplete in ordinary usage.

Yes, absolutely.

A word-for-word English-style structure such as I like the smell... does not map directly onto Romanian grammar. The natural Romanian expression uses a plăcea:

• Îmi place mirosul de pâine caldă.

This is the standard, idiomatic way to say it.

So it is best to learn this pattern as a whole:

• Îmi place + singular thing
• Îmi plac + plural things

Then place would become plac.

Examples:

• Îmi place mirosul de pâine caldă. = I like the smell of warm bread.
• Îmi plac mirosurile de pâine și cafea. = I like the smells of bread and coffee.

The pronoun îmi stays the same, but the verb changes to agree with the plural subject.

A rough guide is:

• Îmi ≈ uhm / eem with a very short vowel at the start
• place ≈ plah-che
• mirosul ≈ mee-RO-sul
• de ≈ deh
• pâine ≈ PUH-ee-neh
• caldă ≈ KAL-duh

A few useful notes:

• â and î represent a central vowel that English does not really have
• c before e is pronounced like ch
• ă is a short unstressed vowel, something like a weak uh

So a rough overall pronunciation is:

uhm PLAH-che mee-RO-sul deh PUH-ee-neh KAL-duh

That is only an approximation, but it can help as a starting point.