You had better take an umbrella with you; it looks like rain.

had 是动词 have 的过去式，在这里构成固定搭配。 better 是形容词 good 的比较级形式，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。