Se a porta estiver aberta, podes entrar.

Because after se meaning if, Portuguese often uses the future subjunctive when the condition refers to something that may happen.

So in Se a porta estiver aberta, estiver means something like if the door is open / if the door happens to be open.

This is very common in Portuguese:

• Se tiver tempo, telefono-te. = If I have time, I’ll call you.
• Se chover, ficamos em casa. = If it rains, we’ll stay home.

If you say Se a porta está aberta, that sounds more like you are talking about a real, current situation you are checking right now. It is possible in some contexts, but estiver is the more standard choice for a condition like this.

Estiver is the future subjunctive of the verb estar.

The future subjunctive is used a lot in Portuguese after words such as:

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

For estar, the future subjunctive forms are:

• eu estiver
• tu estiveres
• ele/ela/você estiver
• nós estivermos
• vós estiverdes
• eles/elas/vocês estiverem

In your sentence, the subject is a porta, so it uses the ele/ela form: estiver.

Because estar is used for a state or condition, while ser is used more for an essential characteristic, identity, or something more permanent.

A door being open is a temporary state, so Portuguese says:

• A porta está aberta. = The door is open.

If you used ser aberta, it would usually mean something different, such as being opened in a more passive or general sense, or describing a characteristic in a different context.

So here, estar aberta is exactly what you want for the condition of the door.

Because aberta agrees with porta, and porta is a feminine singular noun.

In Portuguese, adjectives and many past participles used adjectivally agree in gender and number with the noun:

• o livro aberto = the open book
• a porta aberta = the open door
• os livros abertos = the open books
• as portas abertas = the open doors

So aberta matches a porta.

Podes is the 2nd person singular form of poder, so it shows the speaker is addressing tu.

That means the sentence is speaking to one person in an informal way:

• tu podes = you can

In European Portuguese, tu is very common in everyday speech.

If the speaker wanted to be more formal, they might say:

• Se a porta estiver aberta, pode entrar.

Here pode goes with você, o senhor, or a senhora.

Because Portuguese often drops subject pronouns when the verb ending already makes the subject clear.

So instead of saying:

• Se a porta estiver aberta, tu podes entrar

Portuguese normally just says:

• Se a porta estiver aberta, podes entrar

The verb podes already tells you the subject is tu, so the pronoun is unnecessary unless you want emphasis, contrast, or clarity.

Because after the verb poder (can / to be able to), the next verb stays in the infinitive.

So:

• podes entrar = you can enter
• podes sair = you can leave
• podes esperar = you can wait

This is the same basic pattern as in English:

• you can enter
• not you can enters

So podes entrar is the normal structure.

Yes, absolutely.

Both are correct:

• Se a porta estiver aberta, podes entrar.
• Podes entrar se a porta estiver aberta.

The meaning is essentially the same. The difference is mostly one of focus and flow:

• Se a porta estiver aberta, podes entrar. puts the condition first.
• Podes entrar se a porta estiver aberta. starts with the main action.

In speech, both are very natural.

Because the sentence begins with the conditional clause:

• Se a porta estiver aberta = if the door is open

Then comes the main clause:

• podes entrar = you can come in

When the if-clause comes first, it is very common to separate it with a comma:

• Se chover, fico em casa.
• Se precisares, digo-te.

If the main clause comes first, the comma is often omitted:

• Podes entrar se a porta estiver aberta.

So the punctuation here is normal and helpful.

It works in Portuguese generally, but podes strongly points to European Portuguese or other varieties that use tu regularly.

In Brazil, people often prefer você in many situations, so a more typical Brazilian version might be:

• Se a porta estiver aberta, você pode entrar.

The grammar of se ... estiver is still perfectly valid in both varieties. The biggest clue here is really podes, which is very natural in Portugal.

Not necessarily the distant future. It refers to a possible situation, often one that is not yet confirmed at the moment of speaking.

It can mean:

• if the door is open now
• if the door turns out to be open
• if the door is open when you get there

So the future subjunctive in Portuguese is often about a future or uncertain condition, not just something far ahead in time.

That is why it is so common in everyday instructions and conditions like this one.