We should start our grammar with next terms:
The Subject is the noun which the Verb belongs to;
The Agent is the maker of an action.
When we talk about the Subject of a sentence, what object do we usually make as the Subject?
How to convert an active sentence into its passive form:
1. Use the Object of the active sentence as the Subject;
2. Put the helping verb be into the tense which the Subject (= the Agent) of the active sentence does an action in + add the Past Participle;
3. Turn the Subject (= Agent) of the active sentence into a prepositional phrase beginning with by or a possessive pronoun.
How to convert a passive sentence into its active form:
1. Move the Agent inside the by prepositional phrase or the possessive pronoun to the Subject position;
2. Put the Past Participle into the whatever tense the helping verb be originally was.
Extra information:
How can we choose the main verb in a sentence? Some sentences have two main verbs, but it would be very hard to change some of them to passive. Basically, in this:
“John lost his job because he had been arrested by the local police”.
“The John’s job was lost” – no, just doesn’t work. It sounds too strange. So, this one we can change to active easily “The local police had arrested John”, but this one “lost” we can’t really change here.
And the same thing here:
“The team are constructing a new stadium that will open next year”.
“A new stadium is being constructed and will be opened next year” or you could say “A new stadium which will be opened next year is being constructed”– you could put both of these into passive.
If in a sentence are two verb and one of verbs is in a future tense, will it be more important? No, it’s more just a question of…it depends how you feel when you write it. You can have one verb in passive and one verb in active – that’s actually fine.