Acenda velas, a menos que o gerador funcione.

Here acenda is a command form of acender (to light).

In this sentence, it is the form used for:

• você
• o senhor / a senhora

So it is a polite/formal imperative.

In European Portuguese, if you were speaking informally to one person (tu), you would normally say:

Acende velas, a menos que o gerador funcione.

Because Portuguese uses different command forms depending on who you are talking to.

• acende = command for tu
• acenda = command for você / o senhor / a senhora

So the sentence as written sounds more formal or polite.

This is especially worth noticing in European Portuguese, where tu is very common in everyday speech, so acende would often be the more natural choice with friends or family.

Because a menos que normally requires the subjunctive.

So:

• funciona = indicative
• funcione = present subjunctive

After a menos que (unless), Portuguese treats the action as something uncertain, hypothetical, or conditional, so the subjunctive is used:

• a menos que o gerador funcione

This is one of the most important grammar points in the sentence.

Yes, in normal use, a menos que is followed by the subjunctive.

Examples:

• A menos que chova... = Unless it rains...
• A menos que ele venha... = Unless he comes...
• A menos que o gerador funcione... = Unless the generator works...

So if you see a menos que, you should expect a subjunctive verb after it.

It means unless.

Literally, it is something like except if not or unless it happens that, but the natural English translation is simply unless.

So the structure is:

• main instruction/action + a menos que
  • subjunctive clause

In this sentence:

• Acenda velas = Light candles
• a menos que o gerador funcione = unless the generator works

Because velas is being used in a general, indefinite way.

Acenda velas means something like:

• light candles
• light some candles

It does not mean a specific set of candles.

If you wanted to refer to specific candles, you would more likely say:

• Acenda as velas = Light the candles

So:

• velas = candles in general / some candles
• as velas = the candles

Because here gerador is understood as a specific generator, not generators in general.

Portuguese very often uses the definite article where English also uses the:

• o gerador = the generator

So the sentence is talking about a particular generator in the situation, for example the generator in the house or building.

Yes. Funcionar is the standard verb for a machine, device, or system meaning:

• to work
• to function
• to operate properly

So o gerador funcione means:

• the generator works
• the generator functions
• the generator operates

For machines, this is a very common and natural verb.

Yes. You can also say:

A menos que o gerador funcione, acenda velas.

This means the same thing.

The difference is mostly one of style or focus:

• Acenda velas, a menos que... starts with the instruction
• A menos que..., acenda velas starts with the condition

Both are grammatically correct.

The comma separates the main clause from the conditional exception clause introduced by a menos que.

So:

• Acenda velas = main clause
• a menos que o gerador funcione = subordinate clause

This punctuation is natural and standard in this kind of sentence. If you reverse the order, you would also use a comma:

A menos que o gerador funcione, acenda velas.

Yes, mainly because of acenda.

In European Portuguese:

• acenda sounds polite/formal
• acende sounds informal, used with tu

So the sentence is correct, but depending on who you are speaking to, a more everyday European Portuguese version might be:

Acende velas, a menos que o gerador funcione.

The rest of the sentence, especially a menos que o gerador funcione, is completely natural.