SHOULD

Should + perfectThe aspect expressing completion. E.g.: He’s finished university. infinitive can be used to emphasisSpecial importance or attention given to something. E.g.: All I wanna do is have some fun.e unlikelihood in the past. This use is rather formalNot belonging to colloquial or everyday speech; official. E.g.: I demand that Mr Smith call the person on whom the committee has decided. or may even sound archaic to some speakers.

I bought some sandwiches in case they should not have eaten anything.
Should the plane have crashed, the President would still have survived.

WERE TO

In hypotheticalUnreal, imagined. past situations we can use were to + perfect infinitive to express that the eventA general term expressing that somebody or something does something or that something happens. E.g.: An apple fell on Isaac’s head. was very unlikely to happen.

If he were to have asked you to marry him, what would you have said?
If the plane were to have crashed, the President would have died.

Unlikelihood with modals
Unlikelihood with modals Modals expressing present and future unlikelihood