Questions & Answers about Остальные уже сидят в зале.
Остальные means the others, the rest, or the remaining ones.
In this sentence, it is being used on its own, without a noun. Russian often does this when the noun is understood from context.
For example:
- остальные люди = the other people / the rest of the people
- остальные = the others / the rest
So Остальные уже сидят в зале means something like The others are already sitting in the hall.
It is plural because it refers to more than one person.
The verb also shows this:
- сидят = they are sitting
So the sentence structure is:
- Остальные = plural subject
- сидят = plural verb form
If it were singular, you would expect something like:
- остальной сидит — but this is much less natural by itself when talking about people
- more commonly: остальной человек or a different phrasing would be used