If …should
Sometimes we use should in the if-clause to suggest that something is very unlikely. Note that would is not used in this case.
If you should see James, tell him that he owes me 10 dollars. (You are not very likely to see James, but if you do, tell him that he owes me 10 dollars.)
If he should be late, we will have to go without him. (He is not very likely to be late, but if he is, we will have to go without him.)
If … happen to
If … happen to has a similar meaning to if … should.
If you happen to see James, tell him that he owes me a drink.
Should and happen to can be used together.
If you should happen to pass a supermarket, perhaps you could buy some fresh apples.
Note that we do not use would in the main clause in these structures. Instead, we use will.
If … was / were to
This is another structure used to talk about unreal or imaginary future events
If your Dad were to learn about this, he would throw you out the house.
What would you do, if you were to lose your job?