What do sentences that begin with "If"express?

a possibility

Explanation:

For example:

If it rains, we'll have to cancel out plans.

That sentence starts with a "if" and it represents one of the many outcomes that may happen in the following day, it's not certain but a possibility

Another example:

If I didn't take the bus that day, I would've never met you.

Again, a possibility on what could've happen on that day.

don't enter the room

explanation:

or you can just ask him to move away from his house

study and put some time and effort into your classes and you’ll do great