Question

i cannot allow it- change into passive

Answer 1

it cannot be allowed by me.-here is the given statement in passive voice

Answer 2

Heya,

I cannot allow it----- Active voice

It can't be allowed by me ----- Passive voice

While changing from active to passive the subject becomes object and object becomes subject. 

Here in the given sentence I is subject in active voice while it is subject in passive voice.

Hope it helps u......