Assim que eu entrar no apartamento, vou ligar para você.

Assim que means as soon as.

It introduces an action that will happen immediately after another one:

• Assim que eu entrar no apartamento, vou ligar para você.
• As soon as I enter/get into the apartment, I’m going to call you.

Other Portuguese expressions with a similar meaning are logo que and, in many contexts, quando, but assim que strongly emphasizes immediacy.

Because after assim que when you are talking about a future event, Portuguese normally uses the future subjunctive.

So:

• Assim que eu entrar... = correct
• Assim que eu entro... = not the normal choice for this future meaning

A native English speaker often expects a present tense here because English says as soon as I enter or as soon as I get in, but Portuguese uses a different structure.

Very common triggers for the future subjunctive include:

• quando
• assim que
• logo que
• se

Examples:

• Quando eu chegar, aviso você.
• Se ele vier, nós saímos.
• Assim que ela terminar, me chama.

In this sentence, entrar is the future subjunctive form of entrar.

For many regular verbs, the future subjunctive looks exactly like the infinitive in some persons, especially with eu and ele/ela/você:

• eu entrar
• você entrar
• eles entrarem

That is why it can be confusing.

A quick comparison:

• Infinitive: entrar
• Future subjunctive with eu: quando eu entrar
• Personal infinitive with eu is not used this way here

Some irregular verbs make the difference clearer:

• ser/ir → for
• ter → tiver
• fazer → fizer
• vir → vier

So if the verb were irregular, the pattern would stand out more clearly:

• Assim que eu fizer isso...
• Quando ele vier...

Because entrar em is the usual structure in Brazilian Portuguese for to enter/go into a place.

• entrar no apartamento = enter the apartment / go into the apartment

Here, no = em + o.

So:

• entrar em o apartamento → entrar no apartamento

English speakers often want to use something like to because English says enter the apartment or go to the apartment, but Portuguese uses em with entrar in this kind of sentence.

Compare:

• Vou ao apartamento. = I’m going to the apartment.
• Entro no apartamento. = I enter/go into the apartment.

Both are possible, but vou ligar is much more common in everyday Brazilian Portuguese.

• vou ligar para você = I’m going to call you
• ligarei para você = I will call you

The simple future (ligarei) is correct, but it can sound more formal, more written, or less conversational in many situations.

In natural spoken Brazilian Portuguese, people very often prefer:

• vou ligar
• vou falar
• vou chegar
• vou mandar

So this sentence sounds very natural and modern.

Yes, ligar can have more than one meaning.

Common meanings include:

• to turn on / switch on
• to connect
• to call (someone on the phone)

In this sentence, ligar para você clearly means to call you.

Examples:

• Vou ligar a TV. = I’m going to turn on the TV.
• Vou ligar para você. = I’m going to call you.

The preposition helps:

• ligar algo often means turn something on
• ligar para alguém means call someone

In everyday Brazilian Portuguese, people also often say ligar pra você, with pra instead of para.

Both are possible in Brazilian Portuguese, but they feel a little different.

• vou ligar para você = very clear, very common, neutral
• vou te ligar = common in Brazil, but in some contexts people may also prefer vou te ligar depois or vou ligar para você

A very natural spoken alternative would be:

• Assim que eu entrar no apartamento, vou te ligar.

In Brazil, você is extremely common, and using para você sounds normal and natural.

So this sentence is perfectly fine. It is just slightly more explicit than using te.

Yes, the comma is appropriate here because the sentence begins with a subordinate clause:

• Assim que eu entrar no apartamento, vou ligar para você.

That first part sets the time condition, and then the main clause comes after it.

If you reverse the order, you normally do not need the comma:

• Vou ligar para você assim que eu entrar no apartamento.

So the comma works much like in English when a time clause comes first.

Yes, you often can.

Portuguese frequently drops subject pronouns when the meaning is clear from context:

• Assim que eu entrar no apartamento, vou ligar para você.
• Assim que entrar no apartamento, vou ligar para você.

Both can work.

However, keeping eu can help by:

• making the subject extra clear
• adding a little emphasis
• sounding more explicit, especially if there could be confusion about who is entering

Since entrar in the future subjunctive has the same form for several persons, including eu can be especially helpful.

Yes. That is completely natural.

Both orders are correct:

• Assim que eu entrar no apartamento, vou ligar para você.
• Vou ligar para você assim que eu entrar no apartamento.

The difference is mainly in emphasis:

• Starting with Assim que... emphasizes the time condition first.
• Starting with Vou ligar para você... emphasizes the main action first.

Both are very common.

Both can work, but assim que is more specific.

• assim que = as soon as
• quando = when

So:

• Assim que eu entrar no apartamento, vou ligar para você.
  This strongly suggests the call will happen immediately after entering.

• Quando eu entrar no apartamento, vou ligar para você.
  This can also mean the same thing in many situations, but it does not stress immediacy as strongly.

If you specifically want right after that, assim que is a great choice.