1. I should take the blame for the mistake.
= I (am to blame for) the mistake.
解释：固定句型sb be to blame for sth“某人应该为某事受谴责/负责任”
2. He (joined) the Youth League at the age of 16.
We often (take part in或join in) the after-class activities.
解释：join的宾语是sb或某团体组织,意思是“加入人群/团队中,并成为其中一员”；
take part in和join in一样,其宾语是各种活动,表示“参与某事”；
attend的准确含义是“出席/在场”,一般在其中不起积极作用.