This type 3 conditional expresses a situation in the past that did not happen, so when we use this conditional we are actually imagining a different past that cannot be changed. This unreal situation can never happen, not now or ever.
|Conditional Clause||Main Clause|
|If + past perfect tense||would / could / might|
|If you had watched the match,||you would have known how exciting it was.|
With this conditional, we use the past perfect in the conditional clause and the perfect conditional (would have + past participle) in the main clause to:
1. express disappointment or regret about things that are not possible to happen now.
2. talk about things that should have happened in the past but didn't.
3. imagine unrealistic situations.
4. express criticism.
Other modal verbs such as could and might can also be used in the main clause.