Eu não costumo pedir boleia, a não ser que esteja mesmo sem gasolina.

Costumar + infinitive expresses something that someone usually or habitually does.

So:

• Não costumo pedir boleia = I don’t usually ask for a ride
• Não peço boleia = I don’t ask for rides

The version with costumo sounds more like a general habit or tendency. It is very common in Portuguese when talking about routines and typical behaviour.

In European Portuguese, boleia means a lift / a ride in someone’s car.

So:

• pedir boleia = to ask for a ride / lift
• dar boleia = to give someone a ride / lift

This is the normal word in Portugal. A learner may know carona from Brazilian Portuguese, but in Portugal boleia is the usual choice.

It can be used for both, depending on context.

It may mean:

• asking someone you know to give you a ride
• asking for a ride because you need help getting somewhere
• sometimes even hitchhiking, depending on the situation

In this sentence, it most naturally sounds like asking someone for a lift because you need one, not necessarily standing by the road hitchhiking.

A não ser que means unless or except if.

It introduces an exception to what was said before:

• Eu não costumo pedir boleia = this is the normal situation
• a não ser que... = here comes the exception

It is a very common expression in Portuguese.

Because a não ser que requires the subjunctive.

So after a não ser que, Portuguese normally uses the subjunctive mood:

• a não ser que esteja
• not a não ser que estou

Here, esteja is the present subjunctive of estar.

This happens because the clause expresses a condition or exception, not a simple statement of fact.

The subjunctive is used because a não ser que introduces something uncertain, conditional, or hypothetical.

The speaker is saying:

• normally, I do not ask for a ride
• the only exception would be if I happened to be out of petrol

That exception is not presented as a fact; it is presented as a possible situation. That is exactly the kind of context where Portuguese often uses the subjunctive.

Yes. The subject is still eu, even though it is not repeated.

So the full idea is:

• Eu não costumo pedir boleia, a não ser que eu esteja mesmo sem gasolina.

Portuguese often leaves out subject pronouns when they are already clear from the verb form. Since esteja can fit the context and eu is already established, there is no need to repeat eu.

Mesmo adds emphasis. Here it means something like:

• really
• actually
• completely

So esteja mesmo sem gasolina means not just low on petrol, but really out of petrol.

It makes the condition sound stronger: the speaker would only ask for a ride in a genuinely serious situation.

Because gasolina is being used in a general, uncountable sense.

In Portuguese, with substances and general materials, you often do not use the article in this kind of expression:

• sem gasolina = without petrol / out of petrol
• com água
• sem dinheiro

Sem gasolina is the natural idiomatic way to say someone has no petrol left.

Yes. Literally, estar sem gasolina means to be without petrol.

In natural English, the idea is usually:

• to be out of petrol
• to have run out of petrol
• to be low on petrol in some contexts, though here it sounds more like completely out

Portuguese often uses estar sem + noun for lacking something:

• estar sem dinheiro = to have no money
• estar sem bateria = to have no battery
• estar sem gasolina = to be out of petrol

The comma separates the main statement from the exception clause.

Main statement:

• Eu não costumo pedir boleia

Exception:

• a não ser que esteja mesmo sem gasolina

In writing, this comma is very natural and helps make the sentence easier to read. It is common before phrases like a não ser que when they add a contrasting condition.

Yes, a menos que can also mean unless.

So a similar sentence would be:

• Eu não costumo pedir boleia, a menos que esteja mesmo sem gasolina.

Both are possible. In European Portuguese, a não ser que is very common and natural.

Yes. Pedir boleia without the article is the most natural fixed expression.

You may sometimes hear:

• pedir uma boleia

but pedir boleia is very common and idiomatic, especially when speaking generally.

Compare:

• Vou pedir boleia. = I’m going to ask for a ride.
• Ele deu-me boleia. = He gave me a ride.

Usually gasolina means petrol specifically.

If you want to talk about fuel more generally, Portuguese can use:

• combustível = fuel

So:

• sem gasolina = out of petrol
• sem combustível = out of fuel

In this sentence, gasolina is specific and very natural if the speaker means a petrol-powered car.

Yes, it strongly sounds like European Portuguese because of boleia.

A Brazilian learner would more likely expect carona:

• Portugal: pedir boleia
• Brazil: pedir carona

The grammar of the sentence is fine in both varieties, but the vocabulary points clearly to Portugal.