When we use the passive voice, we're interested in the person / object affected by the action, not in the person / object who does the action.

structure: Subject + passive verb + (by agent)

By agent: the one who does the action

Passive verb: verb to BE (in the same tense as in the active) + Past participle (of the main verb)

1. Passive constructions with an only object:

act: They didn't do it badly.

pass: It wasn't done badly (by them)

If there is a modal verb (must, might...) or an auxiliary verb (will, would...) the passive verb is formed by:

modal/aux v. + BE + past participle

act: They should / will do their homework every day.

Pass: Their homework should / will be done every day.

2. Passive of active sentences with 2 objects (OI and OD)

We tend to make the OI (the person) as the subjetc of the passive sentence.

act: Mary gave me a book for my birthday.

Pass: I was given a book (by Mary) for my birthday.

Verbs that take 2 objects: give, borrow, lend, bring, tell, take, ...

3. Passive construction with infinitive of active sentences with:

a) a general subjetc (they, people)

b) a reported verb (say, think, believe...)

c) a THAT clause

act: People say (that) he is a good actor.

Pass: It is said (that) he is a good actor

He is said to be a good actor