The difference between ''Who'' and ''Whom'' is whom replaces for (whom thay thế cho)

Leslie is the actress (who/whom) stars on the show.

• who
• whom

Gael is the character (who/whose) I said was my favorite.

• who
• whose

Christopher is the person with (who/whom) I discussed the television show. 

• who
• whom

The actress wore diamonds, (that/which) are expensive.

• that
• which

The man ____ broke into our house was caught in two days.

• who
• which
• than
• that

This is the most interesting book ______ I have ever read on this question.

• that
• where
• who
• whom

The girl ____ brother goes with me to the gym is a good singer.

• whose
• that
• when
• whom

We should use "that"

• after preposition

• after superlative

• after determiners (all, any, some, none....)

• after people and things

That is the place......I met my friend Mike.

• where

• which

• that

• who