In English, I know it is fairly common to say things like:
If the postman comes on time, you should give him this.
If you don’t come back within three days, I’m afraid everyone you love must be killed!
If you start throwing up, you must call the doctor!
I know that if the last parts of each sentence were taken as their own sentences, Ancient Greek would generally use imperatives instead of must and should. However within the conditional clauses, I am not too sure.