Question

change active to passive they knew this

Answer 1

that was known by them

Answer 2

Before beginning let us first know what are active and passive voices:

Active voice - A type sentence where in the subject of the sentence precede the object of the sentence.

Passive voice - A type of sentence wherein the object of the sentence precede the subject of the sentence.

Given sentence:

They knew this.

Passive voice of the given sentence:

This was known to them.