Unless means except if. We use unless to make an exception to something we say. Unless is often used in warnings, instead of unless it is possible to say if .... not.

Also See:

In Case


If / Unless / When
a. Theresa can't eat unless her mother lets her.
b. You can't earn more money unless you work harder.
c. You can't understand the lesson unless you listen carefully.

More Examples:

If you aren't tired, let's go out.
Let's go out unless you are tired.

I will call the police or you will leave immediately.
Unless you leave immediately, I will call the police.

Ali must study harder or she won't pass the final exam.
Ali won't pass the final exam unless she studies harder.

If Not / Unless Conversion

You will stay home if you don't come with us.
You will stay home unless you come with us.

You can't win the lottery if you don't buy a ticket dad.
You can't win the lottery unless you buy a ticket dad.

I will trash your sandwich if you don't want to eat it.
I will trash the sandwich unless you want eat it.

I am not going to the party if Carol is not going.
I am not going to the party unless Carol is going.
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