We haven't had dinner yet.

haven't 是 have not 的缩写形式，表示"还没有"。 had 是动词 have 的过去分词形式，在这里与助动词 have 构成现在完成时，表示"已经做过某事"或"还未做过某事"。 haven't had 合起来表示"还没有做过某事"，强调从过去到现在都未完成某个动作。