Unreal conditionals refer to situations which are contrary to facts, plans or expectations. We use the past tenses in the if-part because they work like a double tool:
The sentence on the left refers the past. The sentences on the right refer to the present - the Past Tense indicates that the situation is not real.
In the Conditional Clauses we move one tense back to indicate unreality:
In the main part we can use only three modal verbs:
To talk about past unreal situations we also need to move one tense back:
The past perfect tenses stay the same.
To talk about imaginary past results we use modal perfects:
Unreal past situations can also have imaginary results in the present:
Or in the future: