Когда мы вошли в вагон, у окна было только одно свободное сиденье.

When we got into the carriage, there was only one free seat by the window.

Breakdown of Когда мы вошли в вагон, у окна было только одно свободное сиденье.

• быть: to be
• войти: to enter
• мы: we
• у: by

Questions & Answers about Когда мы вошли в вагон, у окна было только одно свободное сиденье.

Why is there a comma after вагон?

Because Когда мы вошли в вагон is a subordinate clause introduced by когда meaning when. In Russian, a subordinate clause is normally separated from the main clause with a comma.

So the sentence is structured like this:

• Когда мы вошли в вагон = when we entered the carriage
• у окна было только одно свободное сиденье = there was only one free seat by the window

Why is the verb вошли used here, not входили?

Вошли is the perfective past form, so it presents the action as completed: we entered / we went in.

That fits this sentence because the speaker means:

• first, we entered
• then, they noticed there was only one free seat

If you used входили, that would be imperfective, which would sound more like:

• we were going in
• we used to go in
• we were in the process of entering

So вошли is the natural choice for a single completed event.

Why does вошли end in -ли?