unless and if not

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.)