A plataforma fica cheia quando o metrô está atrasado, então eu prefiro chegar cedo.

In this sentence, ficar suggests a change of state: the platform gets/becomes crowded when the subway is delayed.

• A plataforma fica cheia = The platform gets crowded / becomes full
• A plataforma está cheia = The platform is crowded / is full at a particular moment

So fica cheia is a natural choice because it describes what usually happens when there is a delay.

Because cheia agrees with a plataforma, which is a feminine singular noun.

• a plataforma → feminine singular
• adjective must match → cheia

Compare:

• o vagão está cheio = the train car is full
• a plataforma está cheia = the platform is full

Here, plataforma means the station platform, the place where people wait for the train or subway.

In Brazilian Portuguese, plataforma is the normal word for a transit platform.

In Portuguese, using the definite article with nouns is much more common than in English.

So:

• o metrô está atrasado = the subway is delayed

Leaving out the article here would sound unnatural. Portuguese often says the subway, the bus, the car, etc., even when English might be more flexible.

It can mean either one depending on context.

In this sentence, o metrô está atrasado can be understood as the subway/train is delayed. In everyday Brazilian Portuguese, metrô often refers generally to the subway service or a subway train.

So learners should not worry too much about forcing a strict distinction here; context usually makes it clear.

Estar atrasado is a very common way to say that something is late or delayed.

• o metrô está atrasado = the subway is delayed / late
• eu estou atrasado = I am late

The verb estar is used because this is a temporary condition, not a permanent characteristic.

Quando means when.

It introduces the time clause:

• quando o metrô está atrasado = when the subway is delayed

So the structure is:

• main idea: A plataforma fica cheia
• time condition: quando o metrô está atrasado

Então means so, therefore, or then, depending on context.

Here it connects the two ideas:

• the platform gets crowded when the subway is delayed,
• so I prefer to arrive early.

So in this sentence, então is best understood as so or therefore.

Yes, Portuguese often drops subject pronouns, so prefiro chegar cedo would also be correct.

But eu can still be included for:

• emphasis
• clarity
• contrast
• natural rhythm

So:

• prefiro chegar cedo = I prefer to arrive early
• eu prefiro chegar cedo = same meaning, but with a little more emphasis on I

After preferir, Portuguese very often uses an infinitive when the subject stays the same.

• eu prefiro chegar cedo = I prefer to arrive early

This is the simplest and most natural structure here.

You would use a different structure such as prefiro que... mainly when there is another subject:

• Prefiro que você chegue cedo. = I prefer that you arrive early.

Here, cedo is an adverb meaning early.

It modifies chegar:

• chegar cedo = to arrive early

It does not change form here.

Compare:

• Eu cheguei cedo. = I arrived early.
• É cedo. = It’s early.

A few useful pronunciation notes:

• metrô: stress on the last syllable because of ô → meh-TRO
• está: stress on the last syllable because of á → es-TA
• então: stress on the last syllable, and ão has a nasal sound → en-TOWN (roughly, but nasal)

Also:

• cheia is often pronounced something like SHAY-a in Brazilian Portuguese
• fica sounds roughly like FEE-ka

Yes, and in many contexts gets crowded is actually the most natural English translation.

Literally, cheia means full, but with places like platforms, buses, trains, and rooms, English often prefers crowded.

So these are both good depending on context:

• The platform gets full
• The platform gets crowded

The Portuguese sentence works well for either idea.