A lâmpada do corredor não funciona, mas o interruptor ainda funciona.

Because do is the contraction of de + o.

• de = of / from
• o = the (masculine singular)

So:

• de o corredor → do corredor

This is required in normal Portuguese.
Similarly:

• de a → da
• em o → no
• em a → na

Here, a lâmpada do corredor means the hallway lamp or more literally the lamp of the hallway.

In this sentence, A is the feminine singular definite article, meaning the.

• a lâmpada = the lamp

Portuguese a can have different functions depending on context:

• a = the (feminine singular article)
• a = to (preposition)

Here it is clearly the article because it comes right before the noun lâmpada.

Usually, yes. In standard Portuguese, não normally comes directly before the verb it negates.

• não funciona = does not work
• não é = is not
• não tenho = I do not have

So in this sentence:

• A lâmpada do corredor não funciona = The hallway light bulb doesn’t work

This is the normal and expected placement.

Because funcionar means to work / to function, and it can be used for many things: devices, machines, systems, lights, buttons, switches, plans, methods, etc.

So both of these are natural:

• A lâmpada funciona. = The bulb works.
• O interruptor funciona. = The switch works.

Portuguese often uses funcionar in the same broad way English uses work.

Here, ainda means still.

• mas o interruptor ainda funciona = but the switch still works

It suggests a contrast:

• the bulb does not work,
• but the switch continues to work.

Depending on context, ainda can also mean things like yet, even, or still, but in this sentence still is the best match.

Mas means but in Portuguese.

• não funciona, mas... = doesn’t work, but...

It is not the same as Spanish más meaning more.

In Portuguese:

• mas = but
• mais = more

So:

• mas o interruptor ainda funciona = but the switch still works

Portuguese usually prefers to repeat the verb in sentences like this.

English can say:

• The bulb doesn’t work, but the switch still does.

Portuguese can sometimes avoid repetition in certain contexts, but here the most natural sentence is:

• A lâmpada do corredor não funciona, mas o interruptor ainda funciona.

This sounds complete and clear.
Saying only mas o interruptor ainda would be incomplete, and dropping funciona would generally sound unnatural here.

Yes. Interruptor is a masculine noun, so it takes o.

• o interruptor = the switch

In Portuguese, nouns have grammatical gender, and the article must match:

• a lâmpada — feminine
• o interruptor — masculine
• o corredor — masculine

This is why the sentence has:

• A lâmpada
• do corredor (de + o)
• o interruptor

In many contexts, lâmpada refers specifically to the light bulb. But depending on everyday usage, people may also use it in a broader way when talking about a light.

In this sentence, since it says the switch still works, the likely idea is:

• the bulb/light is not working,
• but the switch is fine.

So a natural interpretation is light bulb or light rather than a table lamp.

A simple approximate pronunciation is:

ah LAM-pah-dah doh koh-rreh-DOR naw foon-syOH-nah, mahz oo een-teh-roop-TOR ah-EEN-dah foon-syOH-nah

A few helpful notes:

• lâmpada has stress on the first syllable: LÂM
• corredor has stress on the last syllable: correDOR
• não sounds nasal, roughly like now said through the nose
• funciona often sounds like foon-syOH-na
• mas may sound like mahz in connected speech

Yes, that is also very natural, but it changes the wording slightly.

• A lâmpada do corredor não funciona = The hallway bulb/light doesn’t work
• A luz do corredor não funciona = The hallway light doesn’t work

Luz means light, while lâmpada is more specifically lamp / bulb. In everyday speech, both can work depending on what exactly you want to emphasize.

Because mas joins two contrasting clauses, and in Portuguese it is normally preceded by a comma.

So this structure is standard:

• A lâmpada do corredor não funciona, mas o interruptor ainda funciona.

It helps mark the contrast clearly:

• one thing does not work,
• another thing still does.