Mâine voi face curățenie în apartament.

Putting mâine first emphasizes the time: Tomorrow, I will clean the apartment / do some cleaning in the apartment.

Romanian word order is fairly flexible, so you could also say:

• Voi face curățenie mâine în apartament.
• Voi face mâine curățenie în apartament.

But Mâine voi face curățenie în apartament is very natural and common, especially when the speaker wants to set the time frame right away.

Voi face is the future tense of a face (to do / to make).

It is made of:

• voi = future auxiliary for I
• face = infinitive/base form of a face

So:

• voi face = I will do / I will make

In this sentence, it combines with curățenie to mean I will do cleaning or more naturally I will clean / do some housework.

Fac means I do / I am doing, so it is present tense.

• fac = I do
• voi face = I will do

Because the sentence clearly refers to the future with mâine, Romanian often uses the future form voi face.

That said, in everyday speech, Romanians can also use the present tense for a planned future action:

• Mâine fac curățenie în apartament.

This is also natural, especially in conversation, but voi face is more explicitly future.

Yes. O să fac is a very common future form in spoken Romanian.

So these are both natural:

• Mâine voi face curățenie în apartament.
• Mâine o să fac curățenie în apartament.

Very roughly:

• voi face can sound a bit more neutral, careful, or standard
• o să fac is extremely common in everyday speech

Both mean the same thing here.

A face curățenie is a very common Romanian expression meaning to clean, to do the cleaning, or to tidy up.

Literally, it is something like to do cleaning:

• a face = to do / to make
• curățenie = cleaning / cleanliness / tidying

Romanian often uses this kind of expression where English might prefer a single verb.

There is also a curăța, which means to clean, but it is often used more for cleaning a specific object or surface:

• Curăț masa. = I’m cleaning the table.
• Fac curățenie în apartament. = I’m doing cleaning / tidying up in the apartment.

So a face curățenie is especially natural for general house cleaning.

Romanian often leaves out subject pronouns when they are not needed, because the verb form already shows the person.

Here, voi face already tells you the subject is I.

So:

• Mâine voi face curățenie în apartament.
• Eu voi face curățenie în apartament.

Both are grammatical, but the version without eu is more neutral and more common.

You would add eu mainly for emphasis or contrast:

• Eu voi face curățenie, nu tu.
• I will do the cleaning, not you.

În means in / inside.

So în apartament means in the apartment.

It is used because the action takes place inside that space. This is the normal preposition for location here:

• în casă = in the house
• în cameră = in the room
• în apartament = in the apartment

Romanian often uses the noun without the definite article in expressions of place when the meaning is general or when the location is simply being named.

So în apartament is natural here.

If you want to be more specific, you could say:

• în apartamentul meu = in my apartment
• în apartamentul lui = in his apartment
• în apartamentul de sus = in the upstairs apartment

You may also hear în apartament in a more general sense, like at home / in the apartment.

So the lack of -ul does not make it wrong or incomplete.

Curățenie is a noun.

It comes from the idea of cleanliness / cleaning / tidiness. In this sentence, it refers to the activity of cleaning up.

That is why Romanian says:

• a face curățenie = literally to do cleaning

This is similar to English expressions like:

• do the cleaning
• do some cleaning

So even though English often uses the verb clean, Romanian commonly uses a noun here.

The most important ones are:

• â in mâine = a central vowel, somewhat like the vowel in roses for many English speakers, but not exactly
• ă in curățenie = a short, neutral uh sound
• ț in curățenie = ts, like the end of cats

A rough learner-friendly pronunciation might be:

• Mâine ≈ MUH-ee-neh or MUHY-neh
• voi ≈ voy
• face ≈ FA-cheh
• curățenie ≈ koo-ruh-tse-NEE-eh
• în ≈ a short uhn with a central vowel
• apartament ≈ ah-par-ta-MENT

These are only approximations, but they can help at first.

Yes, it is completely natural.

Other natural versions include:

• Mâine o să fac curățenie în apartament.
• Mâine fac curățenie în apartament.
• Mâine voi face curat în apartament.
  This is also possible, though a face curățenie is especially common.

The original sentence is clear, standard, and idiomatic.