One of the uses of the past subjunctive as if/as though is to indicate unreality, improbability or doubt in the present. Both forms can be used interchangeably,
although as if is more common.
The verb preceding as if/though can be put to a past tense without changing the tense of the subjunctive:
After as if/though we use the Past Perfect when referring to a real or imaginary action in the past:
Again, the verb preceding as if/though can be put into a past tense without changing the tense of the subjunctive: