Unless means if not. We use it in conditional sentences instead of if not.
Unless can be used with present, past and past perfect tenses. Use unless with present tenses when talking about the future.
You will damage your health unless you stop smoking. = you will damage your health if you do not stop smoking.
Unless she was working late, she would be here. = If she was not working, she would would be here.
I wouldn't have taken Paul unless his car hadn't broken down. = If Paul's car had not broken down I would not have taken him.
Now complete these sentences: