in vain

1. "Which" is a little bit like 的. "The X which Y" is like "Y 的 X." Example: The car which I bought = 我买的车。 2. “Whom" is a question word, like 谁，but we only use it when the person we are asking about is the OBJECT of an action, not the subject of the action. Use "Who" when asking about the subject. Examples: You sent the invitation to WHOM? WHO sent the invitation to you? 3. "While" has several meanings. The most common is "during" or "at the same time." Example: "While I was sleeping, she was working." 我睡觉的时候， 他在工作.

"which" is used for an option or explaining a reason -*Which* sandwich do you want today? -That shirt is comfortable, *which* is why I bought it. -I cannot decide *which* movie to watch. "whom" is used in place of "who" when it is not the subject Subject: *Who* are you going to the movie with? Not the subject: With *whom* are you going to the movie? Subject: *Who* is this present for? Not the subject: For *whom* is this present? "while" is for time (either to say "during" an action, or as a measure time - usually a long time) *While* I was at the store, I bought milk. *While* you are in America, make sure you eat a lot of pizza. It has been raining for a *while.* It has been a *while* since I've talked to you. ------ Hope these help a little