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In the sentence "I saw a bird fly in the yard", why is "fly" used in the present form? Why is it not past? What is the structure? Is there a rule?

Verbs following the verb "to see" are in the base form ("do") or the -ing form ("doing").

You see somebody do something.
You see somebody doing something.

The main verb (in this case see) can be in any tense, but the following verb doesn't change:

I saw a bird fly
I will see a bird fly
I have seen a bird fly
I saw a bird flying
I will see a bird flying
I have seen a bird flying

The same is true for other verbs of perception such as hear, smell, feel.