Speculative conditional sentences, sometimes called unreal conditionals, show unlikely, contrary-to-fact, or impossible conditions. To show distance from truth or reality, English uses past or past perfect tense in the if clause, even for conditions in the present and future.
Unlikely possibilities
If the condition is possible but unlikely in the present or future, use the past tense in the if clause; in the independent clause, use would, could, or might plus the base form of the verb.
The writer does not expect to win the lottery. Because this is a possible but unlikely present or future situation, the if clause uses the past tense.
Conditions contrary to fact
In conditions that are currently unreal or contrary to fact, use the past-tense verb were (not was) in the if clause for all subjects.
The writer is not president, so were is correct in the if clause.
NOTE:Do not use were in the if clause if the condition exists or might exist.
Events that did not happen
In a conditional sentence that speculates about an event that did not happen or was impossible in the past, use the past perfect tense in the if clause; in the independent clause, use would have, could have, or might have with the past participle.
The writer did not save enough money and did not travel to Senegal. This sentence shows a possibility that did not happen.
Aunt Grace was not alive at the time of the graduation. This sentence shows an impossible situation in the past.
Exercise:
Verbs in conditional sentences