The Subjunctive Mood is used in adverbial clauses of comparison (or manner) introduced by the conjunctions as if and as though (the latter is more literary).

If the action of the subordinate clause is simultaneous with that of the principal clause the Past Subjunctive of the verb to be is used; with other verbs, the same meaning is expressed by the Past Indefinite of the Indicative Mood.

She speaks (spoke) about him as if she knew him well.

She greeted him as if he were her brother. (Galsworthy)

Не speaks as if he knew you! (Collins)

Rex got from his seat, as if he held the conference to be at an end. (Eliot)

If the action of the subordinate clause is prior to that of the principal clause the Past Perfect of the Indicative Mood is used.

She speaks (spoke) about him as if she had known him for years.

She flushes as though he had struck her. (Shaw)

She shook hands with him as though they had known each other all their lives… (Trollope)