The 'past tense' you're refering to is the subjunctive mood. You mainly use it for facts that are not real eg. if I were you (I am not you!). If a fact is possible (at least in the speaker's opinion), the present indicative is often preferable.
He boasts as if he had won the superbowl.
She behaves as if she were born yesterday.
Gary looks as if he is up to something.
My friend often speaks to his dog as if it understands him (he probably do believe his dog understands).
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