You had 就是说你最好怎样怎样. 这句话念快时 You had 会念成 You'd 或是乾脆就把 had 省略, 例如有时会听到他们这么说 "You better behave." 就是说你最好行为检点一些, 这时就不发 had 的音.
或许你会问, 为什么这里是用过去式 had 而不是 have 呢? 关于这点我也曾觉得奇怪, 但是我请教过老美, 他们说这里的确是用 had 没错, 至于原因, 他们自己也不知道, 只知道从小父母就教他们说 you had better, 而不是 you have better.