Read the following sentences:

1. Active: Does Saritha like ice-cream?
    Passive: Is ice-cream liked by Saritha? (NOT Do ice cream liked by the children?)

2 Active: Did you heat the water?
   Passive: Was the water heated by you? (NOT Did water heated by you?)

Note The auxiliaries do, does and did are not used in the passive.

Active:Have you completed your homework?
Passive: Has your homework completed by you?

Note: The auxiliaries has, have and  had were used in the passive, but they change their form according to the number and person of the passive subject.

Active:    Can we break it?
Passive: Can it be broken?

Active:    Will he invite us?
Passive:  Will we be invited by him?

Note:The auxiliaries can, may, will and shall do not change their beginning position when active voice is changed into the passive voice.

Active:    Whom did you laugh at?
Passive:  Who was laughed at by you?

Active:     Who bought the new car?
Passive:   By whom was the new car bought?

Note: Who changes to, "by whom"  in the passive: (by) whom changes to who in the passive.

Active: Why did he bring the cabs?
Passive: Why were the cabs brought by him?

Note:
The question words when, where, how, why  and what do not change their beginning position when the sentence is changed from the active to the passive.