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。