Mixed conditionals combine different time references across the if-clause and the main clause. They express how a past event affects the present, or how a permanent trait affected a past outcome.
Use if + past perfect in the if-clause with would + base verb in the main clause to show how a past action affects the present.
Use if + past simple in the if-clause with would have + past participle in the main clause to show how a permanent trait affected a past outcome.
Ask yourself: Is the if-clause about a past event or a permanent trait? Then check: Is the result about the present or the past? This determines which mixed pattern to use.