Rano w metrze jest dużo ludzi, ale wieczorem metro bywa prawie puste.

Because the noun is doing two different jobs.

• w metrze means in/on the subway, and after w in this meaning Polish uses the locative case
• metro in metro bywa prawie puste is the subject of the clause, so it stays in the nominative

So:

• metro = nominative
• w metrze = locative

This is also a good reminder that metro does decline:

• metro
• w metrze
• metrem

Because Polish treats expressions with quantity words like dużo, mało, and wiele differently from ordinary plural subjects.

With dużo, you normally get:

• dużo + genitive
• a singular verb in neutral statements

So the normal pattern is:

• jest dużo ludzi = there are a lot of people

Not:

• są dużo ludzie ❌

A helpful way to think about it is that Polish is building a phrase more like there is a large amount of people than people are many.

Because dużo requires the genitive plural.

• ludzie = people, as a subject form
• ludzi = of people / people after quantity expressions like dużo

So:

• Ludzie są zmęczeni. = The people are tired.
• Jest dużo ludzi. = There are a lot of people.

This is a very common Polish pattern, so it is worth memorizing early.

Bywa comes from bywać, and here it means something like:

• can be
• is sometimes
• tends to be

So:

• metro jest prawie puste = the subway is almost empty
• metro bywa prawie puste = the subway is sometimes / often almost empty

In this sentence, bywa makes the statement sound habitual, typical, but not absolute. That fits the idea of evening conditions better than plain jest.

Here they sound general:

• Rano... ale wieczorem... = In the morning... but in the evening...
• in natural English, probably In the mornings... but in the evenings...

The sentence is describing a usual pattern, not one single day. The use of bywa also supports that general, habitual meaning.

They are just different Polish time expressions.

• rano = in the morning
• wieczorem = in the evening

You do not need a preposition with either one here. They both function adverbially.

A useful thing to know is that wieczorem is historically an instrumental form, but for learners it is easiest to treat it as a standard time expression. Polish has several of these:

• rano
• wieczorem
• nocą
• latem
• zimą

So even though they do not all look alike, they can all answer the question when?

Literally, Polish says in the metro/subway, but in natural English on the subway is often the best translation if you mean riding it.

So:

• w metrze literally = in the subway
• natural English in many contexts = on the subway

Polish does not draw the same in/on distinction as strictly as English does here.

Because metro is a neuter singular noun, and the adjective has to agree with it.

• masculine: pusty
• feminine: pusta
• neuter: puste

So:

• metro bywa prawie puste = correct

If the noun were masculine, you would use pusty instead.

Yes. Polish word order is flexible, and changing it usually changes emphasis, not the core meaning.

The original sentence is neutral and natural:

• Rano w metrze jest dużo ludzi, ale wieczorem metro bywa prawie puste.

Other versions are possible, for example:

• W metrze rano jest dużo ludzi...
• Wieczorem metro bywa prawie puste...

But the original order sounds very normal for an everyday statement. For learners, it is a good model to copy.

Because Polish does not have articles like English a/an and the.

So metro can mean:

• a subway
• the subway
• just subway/metro

The exact meaning comes from context. In this sentence, English naturally uses the subway or just the metro, but Polish does not need a separate word for that.