should have known better

should have known better, should've known better 両方が使われます。基幹部分はknow betterですが、 should have known better （もっと気をつけておけば良かった） でおぼえておくほうがいいでしょう。 基本例文500の一部です。0:27 I should have known better with a girl like you.

The beatles I should have known better subtitulado en español
https://www.youtube.com/watch?v=L0L6tLmMAvQ  0:55 they should've known better.
Trump University
https://www.youtube.com/watch?v=qy63L6ZEaIY  2:34 I should've known you'd turn out to be some kind of a world-class screw-up. 　
Stven Seagal : FIght scene Fire Down Below
https://www.youtube.com/watch?v=t4JZsHi4tFw  1:40 I'm sorry I put myself in the circumstances where I should've known better.
Bill Cosby Insists the Show Must Go On
https://www.youtube.com/watch?v=1FcQZ5wbBok  1:46 she should have known better.
Justine Sacco Apologizes for Offensive South Africa Tweet

参考リンク
http://bridge-english.blogspot.jp/2014/09/blog-post_33.html  そんなことなさらなくてもよかったのにを英語でなんと言うか？

練習問題解答
b-0558 should have known better