Many trees were swept away in the flood.

were 是 be 动词的过去式复数形式，表示被动语态。 swept 是动词 sweep 的过去分词，sweep 的意思是"扫、席卷、冲走"。 away 是副词，表示"离开、远去"。 were swept away 是被动语态结构，表示"被冲走"的意思。