The past tense of collect is collected.
Present perfect is formed with --- have/has + past participle.Because collect is a regular verb the past participle is verb + -ed = collected.So present perfect is have collected or has collected egHe has collected his pay.They have collected the rubbish early today.
"Has collected" is the present perfect tense of "collect" used for third person singular subjects.
The past tense of "exist" is "existed." The past perfect tense is "had existed."
If you meant what is collect in past tense, It is collected.
The past tense of sit is sat. The past perfect tense of sit is had sat.
The past tense of "bleed" is "bled." The past perfect tense of "bleed" is "had bled."
The past perfect tense is had set.
The past perfect tense is had rung.
The past perfect tense is had bathed.
The past perfect tense of "confront" is "had confronted."
The past perfect tense of "learn" is "had learned."