Very often, we can use unless to mean if .. . not.
- Come tomorrow if I don’t phone / unless I phone.
I ‘II take the job if the pay’s not too low/ unless the pay’s too low.
We cannot always use unless instead of if not. It depends on the sense.
a. The sentence says A will happen if B does not stop it.’ We can use if not or unless.
- I’ll come back tomorrow if there’s not a plane strike.
(OR . . . unless there’s a plane strike)
Let’s have dinner out — if you’re not too tired.
(OR . . . unless you’re too tired)
b. The sentence says ‘A will happen because B does not happen’. We can use if not, but not unless.
- I’ll be glad if she doesn’t come this evening.
(NOT I’ll be giad unless she comes this evening,)
She’d be pretty if she didn ‘t wear so much make-up.
(NOT . . . unless she wore so much make up.)