Deixe a mochila ali, porque aí ninguém vai cair.

Deixe is the imperative form used with você in standard Portuguese.

• deixar = to leave
• deixe = leave! / let...! when speaking to você

In Brazilian Portuguese, commands for você use the same form as the present subjunctive, so deixar → deixe.

Example:

• Deixe a mochila ali. = Leave the backpack there.

By contrast, deixa is associated with tu in traditional grammar, although in Brazil people also very often say deixa in casual speech even when they mean você.

Yes, deixe sounds correct, natural, and slightly more careful or polite.

In everyday Brazilian speech, many people would also say:

• Deixa a mochila ali...

So the difference is roughly:

• deixe = standard, neutral, a bit more polite or instructed
• deixa = very common in casual spoken Brazilian Portuguese

Both can be heard, but deixe is a very normal textbook and real-life form.

Yes. The subject is understood rather than stated.

In this sentence, the hidden subject is most likely você:

• (Você) deixe a mochila ali...

Portuguese often leaves subjects unstated when the meaning is clear from the verb form or context. In commands, this is especially common.

Portuguese uses definite articles more often than English does.

So where English might say:

• Leave the backpack there. or sometimes just
• Leave your backpack there.

Portuguese naturally says:

• Deixe a mochila ali

Even when English would sound fine without an article in some contexts, Portuguese often prefers one. Here a mochila simply means the backpack.

Both relate to location, but they are not exactly the same.

A simple way to think of them is:

• aqui = here
• aí = there, near the person being spoken to / in that place / then
• ali = over there, a little farther away

In real Brazilian Portuguese, the exact distance can be flexible, but the basic contrast is still useful.

In this sentence:

• ali points to a place where the backpack should be left
• aí can mean something like then, that way, or in that situation

So they are doing different jobs.

Because they express two slightly different ideas.

• ali = the physical location: leave the backpack over there
• aí = the consequence or resulting situation: then / that way nobody will fall

So porque aí ninguém vai cair is very natural Brazilian Portuguese and means something like:

• because then no one will fall
• because that way no one will trip

Here aí is not just another location word. It helps link the first action to the result.

Because porque as one word is the conjunction meaning because.

In this sentence:

• porque aí ninguém vai cair = because then nobody will fall

Very quickly:

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

So here the correct form is porque.

Because ninguém already makes the sentence negative.

• ninguém vai cair = nobody will fall

In standard Portuguese, when ninguém is the subject, you do not need não.

Compare:

• Ninguém vai cair. = Nobody will fall.
• Eu não vou cair. = I will not fall.

So ninguém itself is enough here.

Vai cair is the very common ir + infinitive future construction.

• vai = goes / is going
• cair = to fall

Together:

• vai cair = is going to fall / will fall

In Brazilian Portuguese, this structure is extremely common in everyday speech and often sounds more natural than the simple future:

• cairá

So ninguém vai cair is the normal conversational way to say nobody will fall.

Literally, cair means to fall. But in this context, English speakers often understand it as trip or fall over.

If a backpack is left in the way, the idea is:

• someone may trip over it and fall

Portuguese often uses cair for the result, not necessarily the exact movement that caused it. If you wanted to be more specific, you could say:

• tropeçar = to trip

But ninguém vai cair sounds perfectly natural here.

Yes, depending on the exact meaning you want.

For example:

• Deixe a mochila ali, porque aí ninguém vai tropeçar.

That means:

• Leave the backpack there, because then nobody will trip.

The difference is:

• cair focuses on falling
• tropeçar focuses on tripping

In everyday speech, cair can still work well if the main concern is that someone might fall because the backpack is in the way.

They help show stress and pronunciation.

• aí has stress on the i sound: a-Í
• ninguém has stress on the last syllable: nin-GUÉM

The accent in aí also helps show that the vowels are pronounced separately, not as one blended sound.

In ninguém, the ending is nasal, which is important in Portuguese pronunciation.

Yes, and it would still sound natural, but the structure changes slightly.

Compare:

• Deixe a mochila ali, porque aí ninguém vai cair.
  = Leave the backpack there, because then nobody will fall.

• Deixe a mochila ali para ninguém cair.
  = Leave the backpack there so that nobody falls.

The first one explains the reason/result more conversationally.
The second one expresses purpose more directly.

Both are good Portuguese, but the original sentence sounds very natural in spoken Brazilian Portuguese.