Se o porta-malas estiver cheio, eu deixo a mochila no banco de trás antes de entrar na garagem.

Because after se meaning if, Portuguese often uses the future subjunctive when the condition refers to a future possibility.

So:

• Se o porta-malas está cheio = if the trunk is full (more like a current factual situation)
• Se o porta-malas estiver cheio = if the trunk is full / if it turns out to be full (future or possible situation)

This is very common in Portuguese:

• Se eu tiver tempo... = If I have time...
• Se ele chegar cedo... = If he arrives early...

In this sentence, the speaker is talking about what they will do in a possible future situation, so estiver is the natural choice.

Estiver is the future subjunctive of estar.

Here are some forms of estar in the future subjunctive:

• eu estiver
• você/ele/ela estiver
• nós estivermos
• vocês/eles/elas estiverem

It is commonly used after words like:

• se = if
• quando = when
• assim que = as soon as
• logo que = as soon as

Examples:

• Quando eu estiver pronto, eu saio.
• Se ela estiver em casa, eu ligo.

Portuguese often uses the present indicative to talk about:

1. habitual actions
2. planned actions
3. near-future actions

So eu deixo a mochila... can mean:

• I leave the backpack... as a usual action
• or I’ll leave the backpack... in that situation

In English, we often need a future form like I’ll leave, but Portuguese does not always need deixarei.

Compare:

• Se chover, eu fico em casa. = If it rains, I stay home / I’ll stay home.
• Se o porta-malas estiver cheio, eu deixo a mochila... = If the trunk is full, I leave / I’ll leave the backpack...

Yes. Portuguese often drops subject pronouns when the verb already makes the subject clear.

So both are natural:

• eu deixo a mochila no banco de trás
• deixo a mochila no banco de trás

The version with eu may be used for:

• emphasis
• contrast
• clarity

For example:

• Se o porta-malas estiver cheio, eu deixo a mochila no banco de trás, mas ele leva a dele.

Yes, malas literally means suitcases, but porta-malas is a fixed compound noun meaning trunk or car trunk.

A few important points:

• It is masculine: o porta-malas
• It usually stays in this fixed form in the singular
• In Brazilian Portuguese, it is the normal word for a car trunk

So:

• o porta-malas = the trunk
• um porta-malas grande = a big trunk

Even though malas looks plural, the whole expression functions as one noun.

Because cheio agrees with porta-malas, which is masculine.

• o porta-malas → masculine singular
• therefore cheio → masculine singular

Agreement in Portuguese is very important:

• a mochila está cheia = the backpack is full
• o carro está cheio = the car is full
• o porta-malas está cheio = the trunk is full

So the adjective matches the noun it describes.

Here deixar means something like:

• to leave
• to put and leave
• to set down

In this sentence, eu deixo a mochila no banco de trás means the speaker places the backpack on the back seat and leaves it there.

So in natural English, depending on context, you might translate it as:

• I leave the backpack on the back seat
• I put the backpack on the back seat

Portuguese often uses deixar in this broader way.

No is a contraction of:

• em + o = no

So:

• no banco = in/on the seat

And de trás means in the back or rear.

So no banco de trás means on the back seat or in the back seat area.

Related expressions:

• banco da frente = front seat
• banco do passageiro = passenger seat

Even though English says on the seat, Portuguese commonly uses em in this kind of context, giving no banco.

After antes de, Portuguese normally uses the infinitive.

So:

• antes de entrar = before entering / before I enter

This is the standard structure:

• antes de sair = before leaving
• antes de comer = before eating
• antes de dormir = before sleeping

Portuguese does not use a gerund here the way English might use before entering. The infinitive is the normal choice.

The subject is understood to be the same as the subject of the main clause: eu.

So this means:

• before I enter the garage

Portuguese often leaves the subject unspoken with the infinitive when it is clear from context.

If you needed to make the subject explicit, Portuguese could use a different structure, for example:

• antes de eu entrar na garagem

That version is also possible, but the shorter antes de entrar na garagem is very natural when the subject is obvious.

Because na is the contraction of:

• em + a = na

In Brazilian Portuguese, entrar em is a common structure:

• entrar na garagem = enter the garage / go into the garage
• entrar no carro = get into the car
• entrar em casa = go into the house

So na garagem is just the contracted form required by em + a garagem.

Literally, it means enter the garage or go into the garage. In context, because the sentence mentions the trunk and back seat, it strongly suggests a car-related situation, so it can naturally be understood as:

• before driving into the garage
• or before going into the garage

Portuguese often leaves that kind of detail to context instead of making it explicit.

Yes, to some extent. Portuguese allows some flexibility.

For example, these are still natural:

• Se o porta-malas estiver cheio, deixo a mochila no banco de trás antes de entrar na garagem.
• Eu deixo a mochila no banco de trás antes de entrar na garagem se o porta-malas estiver cheio.

The original version is especially clear because it presents:

1. the condition first
2. the action second

That is a very common and natural order.

It can be understood either way depending on context.

Because of se... estiver plus deixo, it may mean:

• a general habit: If the trunk is full, I leave the backpack on the back seat before going into the garage
• a future plan: If the trunk is full, I’ll leave the backpack on the back seat before entering the garage

Portuguese often leaves this distinction to the situation rather than marking it very explicitly.

Yes, coloco could work, but the meaning shifts slightly.

• deixo a mochila no banco de trás = I leave the backpack on the back seat
• coloco a mochila no banco de trás = I put the backpack on the back seat

Colocar focuses more on the act of placing it there. Deixar focuses more on leaving it there.

In this sentence, deixo sounds very natural because the idea is not only placing the backpack, but also leaving it in that location.