why姐係"點解","為什麼"既意思
有d人會係句子既開頭用黎做助語詞~<=不過好少人會咁用
而why not 就係"點解唔得","為何不"既意思
都可以解作同意人地意見
例如:
Paul: "Shall we go out for some air?"
Mary: "Why not?"
"Why" is asking for reason, example like "Why do you do this?" or, "(The reason) why I stop you is because ..."
"Why not" in a sentense is more like a suggestion, e.g. "Why don't we talk about this?" as an answer "why not" equals to "sure" or "I'd love to"