join
1.说“加入(党派、社会、团体等)”用join,不可用join in。例如:He joined the Pupils' Association last year. 说“加入(某种活动)”用join或join in都可以,如Will you join (in) the game? 和We shall be glad to join (in) the discussion。说“加入(某种活动)”而那“活动”用动名词表达,用join in,不可用join。例如:I joined in rowing.
2.join(参加)是一时的动作,所以不可说He has joined the club for five weeks,该说He joined the club five weeks ago或It is five weeks since he joined the club。