Se il serbatoio è quasi vuoto, devo rifornire l’auto di benzina.

Se means if and introduces a condition.

In this sentence, Se il serbatoio è quasi vuoto sets up the situation: if the tank is almost empty...

A learner might also wonder whether quando could be used. It could, but the meaning changes:

• Se il serbatoio è quasi vuoto... = If the tank is almost empty...
• Quando il serbatoio è quasi vuoto... = When the tank is almost empty...

Se is the right choice when you are expressing a condition rather than simply talking about a time.

Italian uses definite articles very regularly, often more than English learners expect.

Here, il serbatoio simply means the tank. The article is required because serbatoio is a masculine singular noun, and in normal Italian you generally do not drop the article in this kind of sentence.

So:

• il serbatoio = the tank
• serbatoio by itself would sound incomplete in this sentence

This is very normal Italian usage.

Because vuoto agrees with serbatoio, not with benzina or auto.

• serbatoio is masculine singular
• so the adjective must also be masculine singular: vuoto

Agreement works like this:

• il serbatoio è vuoto
• la bottiglia è vuota
• i serbatoi sono vuoti
• le bottiglie sono vuote

So quasi vuoto means almost empty, and vuoto is masculine singular because serbatoio is masculine singular.

Quasi means almost.

So:

• vuoto = empty
• quasi vuoto = almost empty

It does not change form, and it goes directly before the adjective it modifies.

Other examples:

• quasi pronto = almost ready
• quasi finito = almost finished
• quasi pieno = almost full

Devo is the first person singular of dovere, meaning must, have to, or sometimes need to.

So:

• devo = I must / I have to

That is why the sentence means that I need to do something.

A quick mini-table:

• devo = I must
• devi = you must
• deve = he/she/it must
• dobbiamo = we must
• dovete = you all must
• devono = they must

Rifornire means to supply, to refill, or to refuel, depending on context.

In this sentence, it means to refuel the car.

It is correct and fairly formal/neutral. In everyday speech, Italians often use other expressions too, such as:

• fare benzina = to get gas / to fill up with petrol
• fare il pieno = to fill up the tank completely

So these are all possible, with slightly different nuances:

• Devo rifornire l’auto di benzina. = I need to refuel the car.
• Devo fare benzina. = I need to get gas.
• Devo fare il pieno. = I need to fill up.

Your sentence is perfectly correct, but a bit more explicit than casual everyday speech.

Because Italian usually shortens la before a vowel. This is called elision.

So:

• la auto becomes l’auto

This is standard spelling.

Other examples:

• l’acqua from la acqua
• l’amica from la amica
• l’Italia from la Italia

So l’auto is just the normal form of la auto.

Because rifornire is commonly used with the pattern:

rifornire qualcuno/qualcosa di qualcosa

That means:

• to supply/refuel someone or something with something

So:

• rifornire l’auto di benzina = to refuel the car with petrol/gasoline

This di is part of the normal construction of the verb.

More examples:

• rifornire il negozio di merce = to stock the shop with goods
• rifornire la macchina di carburante = to refuel the car with fuel

In this sentence, di benzina is the most natural choice because it matches the usual pattern of rifornire.

• rifornire qualcosa di qualcosa is the standard structure

Using con would sound less idiomatic here.

So the best choice is:

• rifornire l’auto di benzina

If you want a more everyday way to express the idea, many speakers would simply say:

• fare benzina
• mettere benzina nell’auto

But with rifornire, di is what learners should expect.

Because the sentence is expressing a general or immediate condition, and Italian often uses the present tense for this kind of idea.

• Se il serbatoio è quasi vuoto, devo rifornire l’auto di benzina.

This sounds like a general rule or a straightforward situation: whenever the tank is almost empty, I have to refuel.

You can use the future in some contexts:

• Se il serbatoio sarà quasi vuoto, dovrò rifornire l’auto.

That sounds more like a future possibility: If the tank turns out to be almost empty, I’ll have to refuel the car.

So the present tense here is natural and very common.

For an American English speaker, yes: benzina usually corresponds to gas or gasoline.

For a British English speaker, it corresponds to petrol.

So:

• Italian benzina = American English gas/gasoline
• Italian benzina = British English petrol

This is a very useful vocabulary point, because learners sometimes confuse gas in English with gas in Italian. Italian gas usually refers to actual gas, not gasoline.

Yes, but they differ a little in tone and frequency.

• auto = car, fairly neutral
• macchina = car, very common in everyday speech
• vettura = car, more formal or technical

So your sentence could also be expressed as:

• Devo rifornire la macchina di benzina.

That would sound very natural in everyday Italian.

They could, but in everyday speech many people would choose a simpler phrasing.

Your sentence is correct and clear, but everyday spoken Italian might more often use:

• Se il serbatoio è quasi vuoto, devo fare benzina.
• Se il serbatoio è quasi vuoto, devo mettere benzina.
• Se il serbatoio è quasi vuoto, devo fare rifornimento.

So the original sentence is grammatically good, but slightly more formal or explicit than what you might hear in casual conversation.

The structure is:

Se + condition, + main clause

So:

• Se il serbatoio è quasi vuoto = if the tank is almost empty
• devo rifornire l’auto di benzina = I have to refuel the car

Inside the sentence:

• il serbatoio = subject
• è = verb
• quasi vuoto = description of the subject
• devo = main verb of obligation
• rifornire = infinitive after devo
• l’auto = direct object
• di benzina = complement showing what the car is being supplied with

This is a very common Italian pattern: a conditional clause followed by a main clause with dovere + infinitive.