We were supposed to arrive at six, but the train was delayed.

were 是 be 动词的过去时复数形式。 supposed 是 suppose 的过去分词，be supposed to 是固定表达，表示"应该、被期望做某事"。 to 是不定式符号，连接动词原形 arrive。 were supposed to 用于过去时，表示原本按计划/期望应发生但实际未发生的事。