Active voice is when subject is doing the action. For example: I eat porridge. "I" is subject and I am doing something.
On the contrary, passive voice is when something is done to the object. In other words, the target of the action gets promoted to the subject position. For example instead of saying "I eat porridge", I would say "Porridge is eaten by me". The subject of the sentence becomes porridge but it isn't doing something.
The following tenses cannot be changed into passive voice.
Present perfect continuous tense
Past perfect continuous tense
Future continuous tense
Future perfect continuous tense
Sentence having Intransitive verbs
Subject + finite form of to be + Past Participle