why don't+主语+动词；why not一定要直接跟动词，动词前不能有其他成分。why don't结构可以有时态的变化，比如变为过去why didn't，完成式why haven't等，why not没有时态变化。why not可以独立成句，而why don't不可以。
这两个句式,在表达"建议"时,是一样的意思.既"(你)为什么不做.....?"或"(你)应该做.....".如:
Why don't you turn to the reference book?
Why not turn to the reference book?
"(你)为什么不看参考书呢?"也就是"你应该看一下参考书".
但是Why don't you do还可以作为一个疑问句使用,提出疑问.如:
Why didn't you see the football match?
"你为什么没有看这场足球赛?"
why don't you ...? and why not...? are the same.
why not...? is short for why don't you...?they are used to give people suggestions.
why don't you ...? is a formal way.and why not is often used in oral English.
区别在三点：
1、两个句子结构不同
why don't + 主语 + 动词；why not一定要直接跟动词，动词前不能有其他成分
2、why don't 结构可以有时态的变化，比如变为过去why didn't，完成式 why haven't 等，why not没有时态变化
3、why not可以独立成句，而why don't不可以。
其实why not就是why don't you的省略形式.学英语的人应该都知道,在英语中,省略句要省就是主谓一起省.只不过前者在口语中更为广泛的使用,且可以独立成句.而后者则可以有时态的变化.
Why not和Why didn't区别从时态上看，why not 一般现在时，表示现在还没发生的事情，还有改变的机会。而why didn't是过去时态，已成定局了。
从含义上看，why not 和 why don't you 是近义句，都表达提议做某事。而Why didn't只是为什么当初没有做某事。
