You're travelling to another country with your boyfriend. When you got to the airport, he wasn't able to find his passport. You both got worried, but after searching for a few minutes he found it. You're worried that he'll lose it again, so you off this warning.
If you lose that, we're screwed.
This kind of clause is often called a "first conditional". You use this to talk about things that you think might happen in the future:
If you have any problems, call me.
If we don't finish on time, there will be hell to pay.
The result, which comes after the comma, can be all kinds of clauses, including commands, statements, and questions:
If I'm not awake by eight, wake me up.
If it's nice out tomorrow, Holly and I are going to go play a bit of tennis.
If we have a girl instead of a boy, will you be disappointed?
And, of course, you can also reverse the order of the "If" part and the "result" part of the sentence:
We'll be in so much trouble if you lose that thing.
"We're screwed" is a slang expression which means "we're in trouble" or "we have a problem".
The word "screwed" is very casual and slightly rough language. Another phrase with the same meaning which is quite rough is:
Parents usually tell their children not to say this, and it's not allowed to be said on network television. But people (especially younger people) use it in conversation with close friends.
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