AS IF / AS THOUGH

Clauses that start with as if / as though describe an unreal or improbable situation if they are followed by an unreal tense (the past subjunctive or the past perfect subjunctive). Otherwise, they express that the statement is true.

The past subjunctive after as if / as though indicates an unreal situation in the present. However, if the situation is true, we use a real tense to express present time:

He looks as if he knew the answer. (he gives the impression that he knows the answer, but he (probably) doesn't know or we don't know whether he knows or not)
He looks as if he knows the answer. (he knows the answer)

If we put the verb preceding as if / as though into the past tense, the present simple knows changes into past simple, whereas the past subjunctive knew stays the same. Therefore, both sentences will read as follows:

He looked as if he knew the answer.

Consequently, the meaning of this sentence (whether he knew the answer or not) can only be deduced from the context.

The past perfect subjunctive after as if / as though is used to refer to an unreal past situation. If the situation is true, we use a real tense to express past time:

He seems as if he hadn't slept for days. (it seems that he hasn't slept for days, but he (probably) has or we don't know whether he has or not)
He seems as if he hasn't slept for days. (he hasn't slept for days)

If the preceding verb is put into the past tense, the present perfect hasn't slept changes into past perfect, while the past perfect subjunctive hadn't slept stays the same:

He seemed as if he hadn't slept for days.