Se o trem estiver lotado, eu vou pegar o ônibus.

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

So:

• Se o trem estiver lotado... = If the train is crowded/full... in the sense of a future possibility
• Se o trem está lotado... would sound unusual for this meaning

A very common pattern is:

• Se + future subjunctive, main clause in the future / near future / imperative

Examples:

• Se chover, eu fico em casa.
• Se ele chegar cedo, nós vamos sair.

So in your sentence, estiver is there because the speaker is imagining a possible future condition.

Estiver comes from the verb estar.

The verb estar means to be in the sense of a temporary state or condition.

In this sentence, estiver is the future subjunctive form of estar for ele/ela/você.

Here is the relevant conjugation:

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

Since o trem is treated like ele, the sentence uses estiver.

Because lotado describes a condition or state: the train is full/crowded at that moment or in that situation.

Portuguese often uses:

• ser for more permanent or defining characteristics
• estar for temporary states or conditions

So:

• O trem está lotado. = The train is crowded/full.
• O trem é lotado would usually not be the normal way to express this idea

Using estar makes sense because a train is not inherently crowded all the time; it is crowded in a particular moment.

Lotado usually means full, packed, or crowded.

With transportation, it often suggests there are a lot of people and not much space.

So:

• trem lotado = a crowded train / a packed train
• ônibus lotado = a crowded bus

It can also be used in other contexts:

• O restaurante está lotado. = The restaurant is packed.
• A praia está lotada. = The beach is crowded.

Notice that the ending changes for gender and number:

• lotado = masculine singular
• lotada = feminine singular
• lotados = masculine plural
• lotadas = feminine plural

Here it is lotado because o trem is masculine singular.

In Brazilian Portuguese, ir + infinitive is extremely common for talking about the future.

So:

• eu vou pegar o ônibus = I’m going to take the bus / I will take the bus

This is often more natural in everyday speech than the simple future:

• eu pegarei o ônibus

Both are grammatically correct, but vou pegar sounds more conversational and common in spoken Brazilian Portuguese.

Here, pegar o ônibus means to take the bus or to catch the bus.

The verb pegar has many meanings depending on context, such as:

• to take
• to catch
• to grab
• to pick up

With transportation, pegar is very common:

• pegar o ônibus = take/catch the bus
• pegar o metrô = take the subway
• pegar um táxi = take a taxi

So even though pegar literally has a broad meaning, in this context it is the normal everyday choice.

Usually, in Brazil, pegar o ônibus is more natural than tomar o ônibus.

With transportation, Brazilians often say:

• pegar o ônibus
• pegar o metrô
• pegar um táxi

You may also hear tomar with some means of transportation in certain contexts or regions, but for a learner of Brazilian Portuguese, pegar is the safest and most common choice here.

Yes, eu can be omitted.

Portuguese often leaves subject pronouns out when the verb already makes the subject clear.

So both of these are fine:

• Se o trem estiver lotado, eu vou pegar o ônibus.
• Se o trem estiver lotado, vou pegar o ônibus.

Including eu can add emphasis, clarity, or contrast.

For example, if you want to stress I in contrast to someone else:

• Se o trem estiver lotado, eu vou pegar o ônibus, mas ela vai esperar.

So in your sentence, eu is optional, but perfectly natural.

Portuguese uses definite articles more often than English does.

So where English might say:

• If the train is crowded, I’ll take the bus

Portuguese naturally says:

• Se o trem estiver lotado, eu vou pegar o ônibus.

Even when English has no article or uses one more loosely, Portuguese often includes it.

Also, each noun has grammatical gender:

• o trem = masculine
• o ônibus = masculine

That is why both use o.

Yes. This is a basic if sentence expressing a condition.

Structure:

• Se o trem estiver lotado = condition
• eu vou pegar o ônibus = result

It means that the second action depends on the first situation.

This kind of sentence is very common in Portuguese when talking about real or possible future situations.

A useful formula is:

• Se + future subjunctive, future expression

Examples:

• Se ele vier, nós conversamos.
• Se eu tiver tempo, vou estudar.
• Se o trem estiver lotado, eu vou pegar o ônibus.

Yes. You can reverse them.

For example:

• Eu vou pegar o ônibus se o trem estiver lotado.

This means the same thing.

When the se clause comes first, Portuguese usually separates it with a comma:

• Se o trem estiver lotado, eu vou pegar o ônibus.

When it comes second, the comma is usually omitted:

• Eu vou pegar o ônibus se o trem estiver lotado.

Yes, it would.

• cheio = full
• lotado = full, packed, crowded

For public transportation, lotado often sounds stronger and more idiomatic because it suggests a lot of people packed into the space.

So:

• Se o trem estiver cheio... = If the train is full...
• Se o trem estiver lotado... = If the train is crowded/packed...

Both are possible, but lotado is especially common for buses, trains, restaurants, beaches, and other crowded places.

Ônibus is pronounced roughly like OH-nee-boos, with stress on the first syllable.

The accent mark ô shows that:

1. the stressed syllable is the first one
2. the vowel has a closed o sound

So the stress is:

• Ônibus

Without going too deep into spelling rules, the accent helps show the correct stress pattern and vowel quality.

In standard Brazilian Portuguese, yes, trem means train.

However, there is a cultural note: in some regions of Brazil, especially Minas Gerais, trem can also be used informally to mean thing, stuff, or that thing in everyday speech.

But in your sentence, o trem clearly means the train.

Portuguese usually does not need an equivalent of then in this kind of sentence.

The conditional relationship is already clear from se.

So:

• Se o trem estiver lotado, eu vou pegar o ônibus.

is complete and natural by itself.

If you really wanted extra emphasis, Portuguese could add words like então in some contexts, but it is usually unnecessary here. The normal, natural sentence does not include it.