B1. Some clauses with ‘if’, use past tense forms to talk about the present and future, for something that has not happened or is not happening.
B2. We use the past tense forms to talk about the future in clauses with ‘if’, for something that we believe or know will not happen.
B3. and to make suggestions about what might happen.
B4. When we are talking about something which did not happen in the past we use the past perfect in the ‘if clause’ and a ‘modal verb’ in the main clause.
B5. If the main clause is about the past we use a modal with have.
B6. If the main clause is about the present we use a present tense form or a modal without have.