Question

1) we use passive forms (passives) when ...a) we don't know who did something b) we know who did something

Answer 1

We use passive forms when we don't know who did something

Answer 2

Passive forms, also known as passives, are used in two main situations:

a) When we don't know who did something: In this case, the focus is on the action or the result of the action, rather than the doer. Passive forms allow us to express this without specifying the doer. For example, "The money was stolen" emphasizes the fact that the money was taken without identifying the person who stole it [[1]](https://dictionary.cambridge.org/ru/%D0%B3%D1%80%D0%B0%D0%BC%D0%BC%D0%B0%D1%82%D0%B8%D0%BA%D0%B0/%D0%B1%D1%80%D0%B8%D1%82%D0%B0%D0%BD%D1%81%D0%BA%D0%B0%D1%8F-%D0%B3%D1%80%D0%B0%D0%BC%D0%BC%D0%B0%D1%82%D0%B8%D0%BA%D0%B0/objects).

b) When we know who did something: Even when we know who performed the action, passive forms can still be used to shift the focus away from the doer and onto the action or the result. This is often done to be more polite or to avoid assigning blame. For example, instead of saying "You broke the vase," we can say "The vase was broken".

It's important to note that passive forms are constructed using a form of the verb "to be" followed by the past participle of the main verb. The subject of the sentence is the receiver of the action, rather than the doer [[8]](https://studylib.net/doc/8611535/testy--grammatika-.).

I hope this clarifies the use of passive forms. Let me know if you have any further questions!