Mucegaiul apare în baie.

Because Romanian usually puts the definite article at the end of the noun.

• mucegai = mold
• mucegaiul = the mold

So -ul is the masculine/neuter singular definite article here. English uses a separate word the, but Romanian attaches it.

Mucegai is a neuter noun.

In Romanian, neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular you get forms like mucegaiul.

Apare is the 3rd person singular, present tense form of the verb a apărea.

So:

• a apărea = to appear / to show up / to occur
• apare = appears

It matches the singular subject mucegaiul.

Because the subject is singular: mucegaiul.

Romanian verbs change depending on the subject, just like English does a little with appears vs appear.

Here the pattern is:

• eu apar = I appear
• tu apari = you appear
• el/ea apare = he/she/it appears

Since mucegaiul is singular, apare is the correct form.

In Romanian, after a preposition like în, you very often use the noun without the definite article when talking about a place in a general or normal-context way.

So:

• în baie = in the bathroom

Even though English usually says in the bathroom, Romanian often does not need the definite article here.

În baia... is possible, but it usually needs something more specific after it, such as:

• în baia apartamentului = in the apartment’s bathroom
• în baia mare = in the big bathroom

It can mean either, depending on context.

• baie = bath
• baie = bathroom

In Mucegaiul apare în baie, it naturally means bathroom, because that is the place where mold appears.

No, Romanian word order is fairly flexible.

This version is a neutral, straightforward order:

• Mucegaiul = subject
• apare = verb
• în baie = place

You could also say:

• În baie apare mucegaiul.

That version puts more emphasis on în baie.

A rough English-style guide is:

moo-cheh-GAI-ool ah-PA-reh uhn BAI-yeh

A few useful notes:

• g in mucegaiul is a hard g, like in go
• ai sounds like the eye sound in English
• ie in baie is pronounced as two sounds, roughly yeh
• î in în is a specifically Romanian vowel with no exact English equivalent

Because în is simply the standard spelling of the Romanian preposition meaning in.

Romanian uses î and â for the same sound, but the spelling depends on position and word history. At the beginning of a word, Romanian normally uses î, so it is written în, not ân.

Yes, it is natural Romanian.

Romanian often uses a apărea for things that start to become visible or present, including unwanted things like mold. Depending on context, English might translate it as:

• appears
• shows up
• forms
• starts to appear

So even if English chooses a slightly different verb, apare is a normal Romanian choice here.