Unless means if not. We use unless in conditional sentences instead of if not.
How to use UNLESS
- Unless is similar in meaning to if not and can be used instead of if not in certain types of conditional sentences. Like if, unless is followed by a present tense, a past tense, or a past perfect tense.
You will damage your health unless you stop smoking. (You will damage your health if you do not stop smoking.)
Unless she had permission from her parents, she wouldn’t travel alone. (If she had permission from her parents, she would travel alone.)
Unless you had suggested it, I wouldn’t have phoned him. (If you hadn’t suggested it, I wouldn’t have phoned him.)
- We cannot use unless in questions:
What will you do if they don’t help you? (NOT: What will you do unless they help you?)
- We don’t use will or would in the clause after unless:
Unless we leave now, we can be late. (NOT: Unless we will leave now, can be late.)