You must be careful with the distiction between the present form, and past tense form. "He talks as if he knew evrything" means "I'm sure he doesn't but he wants to give the impression that he does by talking in a certain way". "He talks as if he knows everything" means "He probably does from the way he talks".
"He talked as if he knew" could be ambiguous. To remove the abiguity in a given context, say "He talked as if he had known", which is not as common. If the context makes for no ambiguity, use the simpler form.
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