What would you do if . . ., Suppose . . ., Imagine . . .
If I were rich, I would buy a bigger house.
If I had a bigger house, I would invite my friends over.
If Jack had been there, he could have prevented the incident.
If I had seen that movie, I would have cried.
Should you have any questions, feel free to contact me.
Had I been there, I might have been able to help.