Por la mañana, el metro va lleno de gente.

In Spain, por la mañana is the normal way to say in the morning.

• por la mañana = in the morning
• por la tarde = in the afternoon/evening
• por la noche = at night/in the evening

En la mañana does exist, but it is much less natural in Spain and is more common in some parts of Latin America.

Here, por does not mean for. In this expression, it means something like during.

So por la mañana literally feels like during the morning, even though the best English translation is simply in the morning.

Because Por la mañana is a time expression placed at the beginning of the sentence.

Spanish often uses a comma after an introductory phrase like this, especially in writing:

• Por la mañana, el metro va lleno de gente.

The comma helps separate the setting from the main statement. In a very short sentence, some writers might omit it, but here it is perfectly natural.

Spanish uses the definite article much more often than English.

So where English might say:

• The metro is crowded or even just
• Metro is crowded in the morning (in a headline-style way)

Spanish normally says:

• el metro

This refers to the metro as the known transport system, or to the metro in general.

It can mean either, depending on context.

In this sentence, el metro is most naturally understood in a general sense: the metro/underground service is crowded in the morning.

But in other contexts, el metro can also refer to a specific metro train.

This is a very common question.

Ir + adjective/participle can describe the state something is in as it goes along, moves, or functions. With transport, va lleno is very natural and idiomatic.

So:

• El metro va lleno de gente = the metro goes along full of people / the metro is crowded

It sounds more dynamic than está lleno.

Compare:

• El metro va lleno de gente = the metro is crowded, especially as a general or ongoing situation
• El metro está lleno de gente = the metro is full of people, more like a simple static description

Both are possible, but va lleno is very natural here.

Because lleno agrees with el metro, which is masculine singular.

• el metro → lleno
• la estación → llena
• los vagones → llenos
• las estaciones → llenas

So the adjective must match the noun it describes.

De gente means with people or more literally full of people.

The structure is:

• lleno de + noun = full of + noun

Examples:

• lleno de agua = full of water
• lleno de coches = full of cars
• lleno de turistas = full of tourists
• lleno de gente = full of people

In English, we would often translate lleno de gente more naturally as crowded.

Because gente is a collective singular noun in Spanish.

Even though it refers to many people, grammatically it is singular:

• Hay mucha gente = There are lots of people
• La gente está cansada = People are tired

So in this sentence, gente is singular in form, even though the meaning is plural.

Most likely, it is expressing a general, habitual situation:

• In the morning, the metro is crowded.

Spanish uses the present tense for regular facts and habits, just like English does:

• Por la mañana, el metro va lleno de gente.
• En verano, hace mucho calor.

So this sentence is probably not about one specific morning, but about what usually happens.

Yes. In fact, Por las mañanas is often even clearer if you want to emphasize a repeated habit.

Compare:

• Por la mañana = in the morning
• Por las mañanas = in the mornings

Since this sentence describes a usual situation, Por las mañanas, el metro va lleno de gente would also sound very natural.

Yes. A few natural alternatives are:

• Por la mañana, hay mucha gente en el metro.
• Por la mañana, el metro está lleno de gente.
• Por la mañana, el metro va muy lleno.

They all express a similar idea, but va lleno de gente is a very idiomatic way to describe crowded public transport.