Explanation

I saw a beautiful bird means میں نے ایک خوبصورت پرندہ دیکھا 

This is a sentence of past indefinite tense.

See is a verb which means دیکھنا

Saw is the second form and seen is the third form of see.

Past Indefinite tense

Active Voice= Subject +  Second Form of Verb + Object.

Passive Voice = Object + was/were + third of verb + by + subject.