knew is an irregular verb 'know' in the past tense form. It is used when describing a situation from a previous time, when the subject (or speaker) was aware of the facts of a certain situation. For example, 'He knew the answer but refused to respond,' or, 'Mike knew the truth but did not admit it.' When used as a noun, knew refers to an awareness or knowledge of a fact.
1. The investigation revealed that he knew all along who was responsible for the crime.
2. After the trial, it became known that she knew more than she had shared with the court.
3. Everyone was surprised to learn that they both knew each other from many years ago.