Let's think about the difference between if and unless
(1) I'll be surprised ( ) an accident. He drives too fast.
① if he doesn't have ② if he has
③ unless he doesn't have ④ unless he has
(2) The teacher will be disappointed ( ) the exam.
① if her student doesn't pass ② if her student will pass
③ unless her student doesn't pass ④ unless her student passes
(3) I'll do the laundry tomorrow ( ).
① unless it doesn't rain ② if it won't rain
③ unless it rains ④ if it isn't rain.
(4) I think I'll go to the party ( ).
① if you don't want me to ② if you will want me to
③ unless you don't want me to ④ unless you want me to
(5) In case of emergency, leave the building as quickly as possible ( ).
① if instructed otherwise ② so instructed otherwise
③ but otherwise instructed ④ unless otherwise instructed
When I was a high school student, I was taught that "unless" is equivalent to "if + not." Which is often the case, but as I learned to speak English, I found that they are totally different, just like "if" and "when" are different.
Let's look at some examples.
(A) I will be happy if I don't fail the test. (OK)
(B) I will be happy unless I fail the test. (hmm..)
To begin with, there is not much to mention about "if + not" It is just a "if" plus "negation" and you should understand it as such.
The word in question here is "unless." This "unless" clause is not so much "if not" as a bunch of "except that --- " clause.
So, if you say "I will be happy unless I fail the test," that sounds like " I will be happy anyway, ( no matter what happens ) but there is an exception that if I fail the test, then I won't be happy, ( other than that, I will be happy, won't be sad, whatever else happens to me )
Let's consider other examples.
(C) He'll stay at thome if he is not invited to the party. (OK)
(D) He'll stay at home unless he is invited to party. (OK)
On the other hand, when you say (D), that means "He will be at home anyway, but there is only an exception that if he is invited there, then he won't be at home. ( Otherwise, he will be at home )
In a nutshell,
If you think "unless" is "if + not", you might make a strange "unless" sentence, but if you see "unless" as "except", then it always works well.