A. When we are talking about Hypotheses (things that we imagine):
A1. use present tense forms after phrases like ‘what if’, ‘in case’ and ‘suppose’ to talk about the future if we think it is likely to happen.
A2. use a past tense form to talk about the future after ‘suppose’ and ‘what if’ to suggest something is not likely to happen.
A3. use modals ‘would’, ‘could‘ for a hypothesis about the future.
A4. use ‘would’ in the main clause and the past in a subordinate clause to talk about the imagined future.
A5. We use modals with have to talk about something that did not happen in the past.