I remember when I taught a Korean guy Chinese,he asked me the question that:why sometimes Chinese people use 吗 to ask question,while sometimes they choose 呢 ?
Actually i'd never thought about that before,as it seems like i was born to know how to use them appropriately. In my opinion, when you ask "is/does/are/did/have.....or not?" and look forward to getting the "yes/no" answers, you should use 吗。 For example: 你吃饭了吗?---是的,我吃过了。 你是学生吗?---不是,我不是学生。 这个东西有用吗?----不,没有用。 你会游泳吗?-----是,我会游泳/不,我不会游泳。
if u ask "when/what/who/how/why......?" in Chinese, you need to use 呢. For instance: 几点去吃饭呢?---8点。 这是什么呢?----我不知道/这是苹果。 谁会游泳呢?-----我会/他会。 电影怎么样呢?-----很好/不错/不好。 为什么他迟到呢?-----他生病了。 Besides,if you ask "when/what/who/how/why...?" without using 呢,it also makes sense. However, if you use 吗 when you ask "when/what/who/how/why...?" ,or use 呢 when you ask "is/are/does/did/have....?", it is not appropriate.
