O gerador funciona, a menos que falhe o disjuntor.

Because a menos que normally requires the subjunctive in Portuguese.

So:

• falhe = present subjunctive
• falha = present indicative
• falhar = infinitive

After expressions like a menos que (unless), Portuguese uses the subjunctive because the action is presented as a possibility, condition, or uncertainty.

So:

• O gerador funciona, a menos que falhe o disjuntor. = correct
• O gerador funciona, a menos que falha o disjuntor. = not correct here

A menos que means unless.

It introduces an exception to what was just said:

• O gerador funciona = the generator works
• a menos que falhe o disjuntor = unless the circuit breaker fails

It is a fixed expression, and it is very commonly followed by the subjunctive.

You can think of it as similar to:

• unless
• except if
• except in the case that

Both are possible, but falhe o disjuntor is perfectly natural in Portuguese, especially in subordinate clauses.

So these are both grammatical:

• a menos que falhe o disjuntor
• a menos que o disjuntor falhe

The version with the verb first can sound slightly more formal, literary, or stylistically smooth. The version with o disjuntor first may feel a bit more straightforward to learners because it matches English word order more closely.

In other words, Portuguese allows more flexibility here than English does.

Because the two parts of the sentence do different jobs.

• funciona is a straightforward statement of fact or general truth: The generator works.
• falhe appears after a menos que, which introduces a possible exception, so the subjunctive is used.

So the contrast is:

• indicative for the main factual statement
• subjunctive for the uncertain or conditional exception

This is a very common pattern in Portuguese.

Disjuntor usually means circuit breaker.

It is the electrical safety device that cuts off the current when there is a problem, such as an overload or short circuit.

So in this sentence:

• gerador = generator
• disjuntor = circuit breaker

Depending on context, this could be a household, industrial, or backup-power setting.

Strictly speaking, falhar means to fail or to malfunction, not simply to trip.

So:

• o disjuntor falha = the breaker fails / malfunctions
• o disjuntor dispara or desarma = the breaker trips

This matters because in technical contexts, trip and fail are not the same thing.

So if the intended meaning is truly unless the breaker fails, then falhe is right. If the intended meaning were unless the breaker trips, Portuguese would more naturally use another verb.

Yes, you can express a similar idea that way:

• O gerador funciona se o disjuntor não falhar.

This means The generator works if the circuit breaker does not fail.

That is close in meaning, but not exactly the same in structure or emphasis.

• a menos que... = focuses on the exception
• se não... = frames it as a condition

In many situations they are interchangeable, but a menos que is the more direct equivalent of unless.

Because Portuguese often uses the definite article where English may or may not.

Here:

• o gerador = the generator
• o disjuntor = the circuit breaker

The sentence is talking about specific, identifiable equipment, so the definite article is natural.

Portuguese uses articles very regularly with nouns, especially when referring to known or specific things.

It is normal and appropriate here.

The phrase a menos que falhe o disjuntor is a subordinate clause adding an exception to the main statement, so a comma helps separate the two parts clearly:

• O gerador funciona, a menos que falhe o disjuntor.

You may sometimes see punctuation handled differently depending on style, but with this kind of sentence, the comma is very common and natural.

Yes. That version is also correct and very natural.

Compare:

• a menos que falhe o disjuntor
• a menos que o disjuntor falhe

The meaning is the same. The difference is mainly word order and style.

For a learner, a menos que o disjuntor falhe may be easier to process because it follows the more familiar subject + verb order.

Normally, it sounds like a general rule or a general statement about how the system works.

• O gerador funciona in the present tense can mean the generator works or the generator is working, depending on context.
• In a technical sentence like this, it usually means something like the generator operates / will operate normally, except in the stated circumstance.

So the present tense in Portuguese often covers both:

• present-time description
• habitual action
• general truth

Context tells you which one is intended.