I had better leave now, or I'll be late for work.

had 是动词 have 的过去式，在这里构成固定搭配。 better 是形容词 good 的比较级，意思是"更好的"。 had better 是一个固定短语，表示"最好做某事"，带有建议或轻微警告的语气。