You'd better put on your warm chothes when you are out.

前面没有说清楚，不好意思。

有虚拟的味道，可能说话的对象现在穿的不多，于现在事实相反，用 had better.当然不是真正的虚拟，只是让你容易记住。表示建议的话，大家后来一直就这么用了。
所以上面那位说固定用法也是对的。

hople all is clear. ^_^

you should

You had better.

you had better
这是固定用法,记住就行