Vou lhe avisar quando eu chegar.

In Brazilian Portuguese, ir + infinitive is the most common way to talk about the future in everyday speech. So vou avisar sounds natural and conversational.

Avisarei is also correct, but it sounds more formal, more written, or more emphatic. In normal spoken Brazilian Portuguese, vou avisar is usually what you will hear.

Here, lhe means something like to you.

It is an unstressed object pronoun. With avisar, Portuguese often uses a structure like avisar algo a alguém = to inform something to someone. In this sentence, the someone is expressed by lhe.

A useful extra point: lhe can also mean to him or to her in other contexts. The exact meaning depends on who is being addressed or talked about.

In Brazilian Portuguese, putting the pronoun before the infinitive is very common: vou lhe avisar.

The version vou avisar-lhe is grammatical, but it sounds more formal and is less common in everyday Brazilian speech. It can also sound more literary or more European Portuguese in tone.

So if you are aiming for natural spoken Brazilian Portuguese, vou lhe avisar is a very good choice.

Yes. Vou te avisar quando eu chegar is very common in informal Brazilian Portuguese.

A rough difference is:

• te: more informal, very common in speech
• lhe: more formal, more careful, or more neutral depending on the speaker

In many parts of Brazil, people mix forms connected with tu and você, so real-life usage is not always perfectly tidy. But for a learner, it is safe to understand te as more casual and lhe as more formal.

Because Portuguese uses the future subjunctive after quando when the clause refers to a future event.

So:

• quando eu chegar = when I arrive / when I get there in the future
• quando eu chego usually sounds more like a habitual meaning, such as when I arrive, I usually...

This is one of the big differences from English. English uses a present form after when for the future, but Portuguese often uses the future subjunctive.

No. In this sentence, chegar is the future subjunctive, not the infinitive.

It just happens that for many verbs, the future subjunctive has the same form as the infinitive in some persons, especially eu and ele/ela/você.

For example:

• infinitive: chegar
• future subjunctive: quando eu chegar, quando você chegar

With irregular verbs, the difference is easier to see:

• quando eu for
• quando eu tiver
• quando eu fizer

So even though chegar looks like an infinitive, here it is functioning as a conjugated verb form.

Not always. You can also say Vou lhe avisar quando chegar.

Brazilian Portuguese often includes subject pronouns more than some other Romance languages, so eu sounds completely normal. Including it can make the sentence a little clearer or slightly more explicit, but it is not strictly required if the subject is already obvious from context.

So both are possible:

• Vou lhe avisar quando eu chegar
• Vou lhe avisar quando chegar

It tells us when the action of avisar will happen.

So the structure is:

• Vou lhe avisar = I will let you know
• quando eu chegar = when I arrive

This means the sentence is about the time of the notification, not the exact content of it.

In other words, it means I’ll let you know when I arrive, not I’ll tell you the words “when I arrive.”