Não saias até que o pintor termine o trabalho na sala.

Because não saias is a negative command addressed to tu.

In Portuguese, negative commands use the present subjunctive, not the normal present indicative. So:

• Tu sais = you leave / you go out — a statement
• Sai! = leave! / go out! — a positive command to tu
• Não saias! = don’t leave! / don’t go out! — a negative command to tu

So in this sentence, saias is not describing what someone does; it is telling someone not to do it.

Yes. Saias is the 2nd person singular form, so it goes with tu.

Portuguese often leaves the subject pronoun out because the verb ending already shows who the subject is. So:

• Não saias... and
• Tu não saias...

mean the same thing, but the version without tu is usually more neutral.

If you changed the person, the verb would change too:

• Não saia... = command to você
• Não saiam... = command to vocês

Because até que normally takes the subjunctive when it refers to something that has not happened yet and marks a future limit: until the painter finishes.

So:

• até que o pintor termine = until the painter finishes

Here the finishing is still in the future from the speaker’s point of view, so the subjunctive is used.

Compare the idea:

• Espero até que ele chegue. = I wait until he arrives.
• Fica aqui até que eu volte. = Stay here until I come back.

In this sentence, termine is the present subjunctive of terminar.

Yes. That is also possible.

Portuguese can express this idea in two common ways:

• até que o pintor termine o trabalho
• até o pintor terminar o trabalho

The first uses até que + subjunctive.
The second uses até + infinitive.

Both can mean until the painter finishes the work. The version in your sentence is very natural and clear.

Sair can mean both to leave and to go out.

Which English translation fits best depends on the situation. In this sentence, leave is often the most natural choice, because the idea is probably don’t leave before the painter has finished.

But depending on context, don’t go out could also work.

So the core idea of sair is to go out / to exit / to leave a place.

Na sala means in the room or in the living room, depending on context.

In European Portuguese, sala often means:

• living room
• sitting room
• more generally, room or hall

If this is about work being done in a home, na sala will often be understood as in the living room.

Because na is a contraction:

• em + a = na

So:

• na sala = in the room / in the living room

This is very common in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

Since sala is feminine singular, the correct form is na.

Portuguese uses definite articles more often than English does.

Here:

• o pintor = the painter
• o trabalho = the work / the job

This usually suggests a specific painter and a specific piece of work that both speaker and listener can identify from the context.

In natural English, you might sometimes translate o trabalho as the job rather than the work, depending on context.

Yes, o trabalho is natural.

Portuguese often states the object explicitly:

• terminar o trabalho = finish the work / finish the job

You could sometimes omit it if the context is obvious, but including it makes the sentence clearer: the painter is not just finishing in general; he is finishing the work he is doing.

They are spelled the same, but here saias is definitely a verb form, not a noun.

• saia / saias can be the noun skirt / skirts
• saias can also be a form of the verb sair

In this sentence, the structure makes it clear:

• Não saias...

After não, this is understood as the verb sair in a negative command: don’t leave / don’t go out.