unhappy life of many people, = many people's unhappy life I know of your father's unhappy marriage = I know about two meanings, 1) possession 2) about

‘In’, ‘with’ and ‘of’ are prepositions used with ‘experience’ having different meanings. ‘Of’ refers to the effect that undergoing an experience has on a person, and often refers to feelings and emotions. He was the only student in the class with the experience of growing up as an orphan. (He knew how it felt) ‘With’ and ‘in’ are similar. ‘With’ is more general. ‘In’ refers to something that is more detailed.

I know of your father's unhappy marriage and how your father and mother separated. vs I know your father's unhappy marriage and how your father and mother separated. "know of" is a phrasal verb so you can't only focus on "of" Can "of" be left out of the sentence? No because they mean different things. "know of" means you are aware of something but you don't have detailed knowledge or information. For example, we can be aware of something or someone, but that doesn't mean we know that person personally or thing deeply. For me, I know of the Chinese language, but I don't know how to speak it. "know" in this sentence means you know on a higher level than just being aware of. Hope this helps.