THAT VS. WHICH – That is used when the information is essential to the noun. Which is used for non-essential information that adds a new element. It requires use of a comma.
Examples:
The movie that opened last week at London Theatre. (It’s not just any movie, it’s the one that opened last week)
The movie, which cost $4 million to make, opened last week at London Theatre. (The amount it cost to make is additional information)
WHO VS. WHOM – Who stands for he, she, or they. Whom stands for him, her, or them.
Examples:
She met a man who she thought was her brother. (she thought he was her brother, so use who)
She met a man whom she took for her brother. (she took him for her brother, so whom is correct)