Se o presente não couber na mochila, eu deixo o pacote no carro.

Because after se when you are talking about a possible future situation, Portuguese normally uses the future subjunctive.

So:

• Se o presente não couber na mochila... = If the gift doesn’t fit in the backpack...

Here, couber is the future subjunctive form of caber.

Compare:

• Se cabe would sound wrong for this kind of future condition.
• Se couber is the standard form for if it fits / if it should fit / if it ends up fitting.

This is a very common pattern in Portuguese:

• Se eu tiver tempo, eu vou.
• Se ele puder, ele ajuda.
• Se chover, ficamos em casa.

Couber comes from the verb caber, which means to fit or to be able to fit inside something.

Examples:

• Não cabe na caixa. = It doesn’t fit in the box.
• Cabe no carro. = It fits in the car.

Caber is irregular, so its forms do not follow the most basic pattern. In this sentence:

• infinitive: caber
• future subjunctive: couber

Other useful forms:

• cabe = it fits
• cabia = it used to fit / it would fit
• coube = it fit / it did fit
• caberá = it will fit

In Portuguese, the present indicative is often used to express a future action, especially in everyday speech, when the future meaning is already clear from the context.

So:

• eu deixo o pacote no carro

literally looks like I leave the package in the car, but here it means:

• I’ll leave the package in the car

Because the first clause already creates a future condition:

• Se o presente não couber na mochila...

the second clause is naturally understood as the result in the future.

You could also say:

• Se o presente não couber na mochila, eu deixarei o pacote no carro.

But that sounds a bit more formal or emphatic.

The sentence starts with a conditional clause:

• Se o presente não couber na mochila

After that comes the main clause:

• eu deixo o pacote no carro

In Portuguese, it is very common to put a comma after an introductory clause like this, especially when it comes before the main clause.

So the structure is:

• If X, then Y.

That is why the comma appears after mochila.

Yes, presente can mean both:

• present / current in the sense of time
• gift / present in the sense of something you give someone

In this sentence, the context makes it clear that presente means gift, because something can fit or not fit inside a backpack.

Examples:

• o presente de aniversário = the birthday gift
• o tempo presente = the present time

So learners often need to rely on context with this word.

Because Portuguese normally contracts the preposition em with definite articles.

So:

• em + a = na
• em + o = no
• em + as = nas
• em + os = nos

That gives us:

• na mochila = in the backpack
• no carro = in the car

These contractions are standard and expected in normal Portuguese.

Portuguese uses definite articles much more often than English.

In this sentence, the speaker is referring to specific, understood objects:

• o presente = the gift
• a mochila = the backpack
• o pacote = the package
• o carro = the car

Even when English might say something like in my backpack or just in the car, Portuguese often keeps the article:

• na mochila
• no carro

This is very natural in Portuguese.

Yes, eu could be omitted.

Portuguese is a pro-drop language, which means subject pronouns are often left out when the verb already shows who the subject is.

So both are possible:

• Se o presente não couber na mochila, eu deixo o pacote no carro.
• Se o presente não couber na mochila, deixo o pacote no carro.

Both mean the same thing.

Why include eu then?

Usually for one of these reasons:

• emphasis
• clarity
• contrast

For example, eu may sound like I will leave it in the car as opposed to someone else doing it.

They are not exactly the same word.

• presente = the gift itself
• pacote = the package, parcel, or wrapped item

In context, the package probably contains the gift, or the speaker is referring to the wrapped gift as a package.

So the sentence suggests something like:

• If the gift itself won’t fit in the backpack, I’ll leave the package in the car.

This kind of shift is normal in real language, especially when a wrapped gift can be thought of both as a gift and as a package.

In Portuguese, não normally comes directly before the verb it negates.

So:

• não couber = does not fit / will not fit

The same pattern appears everywhere:

• não quero = I don’t want
• não cabe = it doesn’t fit
• não vou = I’m not going

That is why you get:

• Se o presente não couber...

not a structure with não after the verb.

No, not in standard Portuguese.

After se in this kind of future condition, you need the future subjunctive, not the infinitive.

So the correct form is:

• Se o presente não couber na mochila...

not:

• Se o presente não caber na mochila...

A useful thing to remember is that many irregular verbs have special future subjunctive forms:

• se eu for
• se eu tiver
• se ele puder
• se o presente couber

Yes, absolutely.

This is also correct:

• Eu deixo o pacote no carro se o presente não couber na mochila.

The meaning stays the same.

The difference is mostly one of focus and style:

• Se o presente não couber na mochila, eu deixo o pacote no carro.
  This emphasizes the condition first.
• Eu deixo o pacote no carro se o presente não couber na mochila.
  This starts with the action/result.

Also, when the se clause comes first, a comma is usually used. When it comes second, the comma is often omitted.

Usually, mochila means backpack or school bag.

In Brazilian Portuguese, it is the normal word for a bag carried on the back with two straps.

Depending on context, it could also be translated more generally as bag if the exact type is not important, but backpack is the most direct translation here.

So:

• na mochila = in the backpack

Yes, it sounds natural.

It uses very normal Brazilian Portuguese structures:

• se
  • future subjunctive: se ... couber
• present indicative with future meaning: eu deixo
• common contractions: na, no
• ordinary everyday vocabulary: presente, mochila, pacote, carro

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

• Se o presente não couber na mochila, deixo o pacote no carro.
• Se não couber na mochila, eu deixo no carro.

But the original sentence is perfectly natural.