Quando a senhora vir o escorregador, vai entender por que a menina quer brincar ali de novo.

Yes, vir is normally the infinitive to come. But in this sentence, vir is not the verb vir. It is the future subjunctive form of ver (to see).

So here:

• quando a senhora vir o escorregador = when you see the slide

This is a very common point of confusion because:

• ver = to see
• future subjunctive of ver = vir
• vir = also the infinitive to come

The meaning is clear from the context. Since the sentence has a senhora vir o escorregador, it must mean see the slide, not come the slide.

Because quando is referring to a future event.

In Portuguese, when a clause with quando, se, assim que, or similar words refers to something that will happen in the future, Portuguese often uses the future subjunctive.

So:

• Quando a senhora vir o escorregador, vai entender...
• literally: When you see the slide, you’ll understand...

Compare:

• Quando a senhora vê o escorregador... = when you see the slide / whenever you see the slide (habitual, general)
• Quando a senhora vir o escorregador... = when you see the slide (at some future moment)

That is why vir appears here.

A senhora is a more formal and respectful way to say you when speaking to a woman in Brazilian Portuguese.

So:

• você = normal, everyday you
• a senhora = polite/formal you, often used with older people, customers, strangers, or in respectful situations

This changes the tone of the sentence. It sounds more polite and formal than:

• Quando você vir o escorregador, vai entender...

Also notice that a senhora takes third-person verb forms, just like você does:

• a senhora vai
• a senhora entende
• a senhora vir

Both are possible, but vai entender is much more common in everyday Brazilian Portuguese.

• vai entender = near-future / conversational future
• entenderá = simple future, more formal or written

So:

• vai entender sounds natural and spoken
• entenderá sounds more formal or literary

Both mean:

• you will understand

This is very common in Brazilian Portuguese:

• vou fazer instead of farei
• vai chover instead of choverá

Because once the subject is already clear, Portuguese often leaves it out.

The full version could be:

• Quando a senhora vir o escorregador, a senhora vai entender...

But repeating a senhora sounds unnecessary here. Portuguese commonly omits the repeated subject when the meaning is obvious from context.

So the sentence naturally means:

• When you see the slide, you will understand...

with a senhora understood as the subject of vai entender.

Here por que means why in an indirect question:

• vai entender por que a menina quer brincar ali de novo
• you’ll understand why the girl wants to play there again

That is why it is written as por que.

A useful quick guide:

• por que = why / for what reason
• porque = because
• por quê = why? at the end of a question
• porquê = the noun the reason

In this sentence, it is not giving a reason with because. It is introducing why she wants to play there again.

Escorregador means slide, the playground object children slide down.

It comes from the verb escorregar, meaning to slip or to slide.

So in context:

• ver o escorregador = see the slide

This word is especially common in Brazil for a playground slide.

Ali means there, usually a place that is not right here, but not imagined as very far away either.

A simple comparison:

• aqui = here
• ali = there
• lá = over there / there, often farther away or less specific

So:

• brincar ali = play there

In this sentence, ali points to that specific place, probably near the slide or in that area.

De novo means again.

So:

• quer brincar ali de novo = wants to play there again

In everyday Brazilian Portuguese, de novo is extremely common for again.

It can mean:

• repeating an action
• doing something one more time

Here it suggests the girl has already played there before and wants to do it again.

These are definite articles:

• o escorregador = the slide
• a menina = the girl

Portuguese uses definite articles very often, sometimes more often than English does. In this sentence, both nouns are specific:

• a specific slide
• a specific girl

So the articles sound natural and expected.

Because Portuguese, like English, often uses a conjugated verb followed by an infinitive.

• quer brincar = wants to play

Here:

• quer is the conjugated form of querer
• brincar stays in the infinitive

This structure is very common:

• quero comer = I want to eat
• ela quer sair = she wants to leave
• a menina quer brincar = the girl wants to play

Brincar can mean simply to play, without naming a toy or another person.

So:

• a menina quer brincar = the girl wants to play

If you want to say play with something/someone, you can add com:

• brincar com a bola = play with the ball
• brincar com os amigos = play with friends

But in this sentence, the focus is just that she wants to play there, so no object is needed.

The comma separates the introductory subordinate clause from the main clause:

• Quando a senhora vir o escorregador, = subordinate time clause
• vai entender... = main clause

This is normal punctuation in Portuguese and also very natural in English:

• When you see the slide, you’ll understand...

The comma helps mark the pause between the two parts.

Not in standard grammar for this meaning.

Because the sentence refers to a future moment, standard Portuguese uses the future subjunctive:

• Quando a senhora vir o escorregador...

Using ver here would sound wrong in standard Portuguese, because ver is the infinitive, not the correct conjugated form for this structure.

So the correct pattern is:

• quando
  • future event → future subjunctive

Examples:

• Quando eu for... = When I go...
• Quando você tiver tempo... = When you have time...
• Quando a senhora vir o escorregador... = When you see the slide...

It breaks down like this:

• Quando a senhora vir o escorregador = When you see the slide
• vai entender = you will understand
• por que = why
• a menina quer brincar ali de novo = the girl wants to play there again

So the structure is:

• time clause
  • main clause
    • indirect question

A very literal map would be:

• When you see the slide, [you] will understand why the girl wants to play there again.

Yes, it sounds natural.

A Brazilian speaker might also say slightly different versions, such as:

• Quando a senhora vir o escorregador, vai entender por que ela quer brincar ali de novo.
• Quando você vir o escorregador, vai entender por que a menina quer brincar ali de novo.

The original sentence is perfectly normal, especially if the speaker wants to sound polite by using a senhora.