Quando eu chegar ao posto, vou colocar gasolina no carro.

Because after quando when you are talking about a future situation, Portuguese normally uses the future subjunctive.

So:

• Quando eu chego... = when I arrive / whenever I arrive, usually for present or habitual contexts
• Quando eu chegar... = when I arrive, referring to a specific future moment

This is very common in Portuguese after words like:

• quando = when
• se = if
• assim que = as soon as
• logo que = as soon as

Examples:

• Quando eu chegar, te ligo.
• Se ele vier, eu aviso.

For chegar, the future subjunctive form for eu is chegar.

Yes, it can be omitted.

Portuguese often drops subject pronouns because the verb form usually makes the subject clear:

• Quando eu chegar ao posto...
• Quando chegar ao posto...

Both are correct.

Including eu can:

• add clarity
• add emphasis
• make the sentence easier for learners to follow

In everyday Brazilian Portuguese, people often omit subject pronouns unless they want emphasis or need to avoid ambiguity.

Because ao posto means to the station, while no posto means at the station.

Here:

• a = to
• o posto = the station
• a + o = ao

So:

• chegar ao posto = to arrive at / reach the gas station
• estar no posto = to be at the gas station

Examples:

• Quando eu chegar ao posto... = when I get to the station
• Eu estou no posto. = I am at the station

With chegar, Portuguese normally uses a in more careful or standard phrasing:

• chegar ao posto
• chegar à escola

In everyday Brazilian speech, you may also hear chegar no posto, which is very common conversationally, but chegar ao posto is more standard.

Yes, in Brazil it often does.

Posto is short for:

• posto de gasolina
• posto de combustível

So if the context is driving or fuel, posto usually means gas station.

Examples:

• Vou parar no posto.
• Tem um posto ali na esquina.

On its own, posto can also mean post or station in other contexts, but in this sentence the meaning is clearly gas station.

Because ir + infinitive is extremely common in Brazilian Portuguese for talking about the future.

So:

• vou colocar gasolina = I’m going to put gas in / I will put gas in
• colocarei gasolina = I will put gas in

Both are grammatical, but vou colocar sounds more natural in everyday spoken Brazilian Portuguese.

In general:

• vou + infinitive = very common in speech
• simple future like colocarei = more formal, literary, or less common in conversation

Yes, it is natural and understandable.

Colocar gasolina no carro literally means to put gasoline in the car, and Brazilians do say this.

Another very common option is:

• abastecer o carro = to fill up / refuel the car

So you might hear both:

• Vou colocar gasolina no carro.
• Vou abastecer o carro.

They are both natural.
Abastecer o carro is a little more specifically tied to refueling.

Because gasolina is being used as an uncountable noun, like water or milk in English.

So:

• colocar gasolina = put gas in
• comprar pão = buy bread
• beber água = drink water

You do not always need an article before these nouns.

You could use an article in other contexts, for example when being more specific:

• A gasolina está cara. = Gas is expensive.
• Essa gasolina é boa. = This gasoline is good.

But in colocar gasolina no carro, no article is needed.

Because no is a contraction of em + o.

• em = in
• o carro = the car
• em o carro becomes no carro

This kind of contraction is very common in Portuguese:

• em + o = no
• em + a = na
• de + o = do
• a + o = ao

So:

• gasolina no carro = gas in the car

Yes.

The sentence can also be said as:

• Vou colocar gasolina no carro quando eu chegar ao posto.

This has the same basic meaning.

Portuguese allows flexibility in clause order, especially with time clauses like quando eu chegar ao posto.

The original version puts the time condition first:

• Quando eu chegar ao posto, vou colocar gasolina no carro.

This is very natural because it sets up the situation first and then gives the action.

They refer to different times.

• quando eu chegar = when I arrive, in the future
• quando eu cheguei = when I arrived, in the past

Examples:

• Quando eu chegar ao posto, vou colocar gasolina no carro.
• Quando eu cheguei ao posto, ele já estava fechado.

So the form chegar here is not random; it is specifically showing a future event after quando.