I was under the impression that you didn't enjoy the party.

was 是 be 动词的过去式，表示"是"或"存在"的状态。 under 是介词，本义是"在...下面"，在这里与 impression 搭配构成固定表达。 the 是定冠词，表示特指某个印象。 impression 是名词，意思是"印象、感觉"。 was under the impression 是固定搭配，表示"有...的印象/以为..."，通常指某种误解或错误认知。