I knew that he had booked a table for two in one of the most expensive restaurants in town, where the food was supposed to be excellent.

was 是 be 动词的过去式，表示"是"的状态。 supposed 是动词 suppose 的过去分词形式，suppose 的意思是"假设、认为"。 to 是不定式符号，用于连接动词原形。 be 是系动词原形，表示"是"。 was supposed to be 是一个固定搭配，表示"被认为是、据说是"，用来表达一种普遍的看法或预期。