"You'd better not go to the party" 是一种建议语气。这句话的来源是英语中的惯用语。
例句：
You'd better not go to the party if you're feepng sick.
(如果你感觉不舒最好不要去参加聚会)
I think you'd better not go to the party tonight.
(我认为你今晚最好不要去参加聚会)
"You'd better not" 这种用法可以用来给对方一个建议或者是警告，强调不建议对方这么做。