After his training, he was posted to Paris for a diplomatic assignment.

was 是 be 动词的过去式，在这里构成被动语态。 posted 是动词 post 的过去分词形式，post 本意是"邮寄、张贴"，引申为"派驻、任命"。 to 是介词，表示方向或目的地。 was posted to 是被动语态结构，表示"被派往/被派驻到"某个地方。