as if vs. verb tense

For the last few days, I asked questions a lot! Appreciating all of you who supported me, I think I have to ask another question again...


In what cases do we have to use the past tense or simple present (continuous) in the sentence following "as if"?
Is there anybody who can show me some examples?

In the sentence below, should the verb tense -- understood-- be always past tense? Or couldn't it be simple present or something else, such as "can understand" or "understands"?

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My friend often speaks to his dog as if it understood him.


THANK YOU! *^^*

[Francois]

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.
Eg.
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).

FRC