ClauseA group of related words containing a subject and a verb.s that start with as if/as though express unrealA tense that expresses a hypothetical situation. E.g.: I wish you were here.ity, improbability or doubt if they are followed by an unreal tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses. (a past subjunctive or a past perfectThe aspect expressing completion. E.g.: He’s finished university. subjunctive). Otherwise, they express that the statement is true.

The past subjunctive is used if we talk about an unreal situation in the present. However, if the situation is true, we use a real tense expressing present timeA concept which is related to our perception of reality; that part of existence which is measured in seconds, minutes, hours, days, weeks, etc.:

He looks as though he knew the answer. (He gives the impression that he knows the answer, but he doesn't know, 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 verbA part of speech that expresses an action or a state. E.g.: John seldom plays tennis. preceding as if/as though into the past tense, the present simple knows changes into past simple, following the sequence of tenses, 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 contextThe situation within which something exists or happens, or an utterance is made..

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

He seems as if he hadn’t slept for days. (He seems not to have slept for days, but he has, he probably has or we don't know if he has or not.)
He seems as though he hasn't slept for days. (He hasn't slept for days.)

In the past tense we get:

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

IT'S (HIGH) TIME
Conditionals and unreal tenses WOULD RATHER, WOULD SOONER