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

In the morning there are many people in the metro, but in the evening the metro can be almost empty.

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

• być: to be
• w: in
• wieczorem: in the evening
• ale: but

Questions & Answers about Rano w metrze jest dużo ludzi, ale wieczorem metro bywa prawie puste.

Why is it w metrze, but later just metro?

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

Why is it jest dużo ludzi and not są dużo ludzie?

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.