My brother has worked as a waiter since last year.

question:how long has your brother worked as a waiter?(你弟弟做服务生做多久了？）
answer:my brother has worked as a waiter since last year（我弟弟从去年就开始做服务生了）
说明：
1）这是一个典型的现在完成时的语态。强调的是一个持续的状态（从去年到现在一直都在做服务生）。
2)强调一个持续状态的时候，疑问句是用“how long”，强调点状态的时候用“when”.比如下面的一疑问句，when did your brother work as a waiter?
这句话是说“你弟弟什么时候做的服务生？”这明显是个点状态。如果把这句当成答案，那回复的时态就不应该是现在完成时，而是一般过去时。

不知道我解释的清楚了没有，请楼主自行比较。注意时态。

How long has you brother worked as a waiter?
对since提问，就是提问时间，所以用how long

When does your brother start to work as a waiter?
since last year.是开始的时间

How long has your brother worked as a wa

How long /When does your brother work as a waiter?

since last year从去年开始
问的时候问问题的人并不知道你的兄弟是从去年开始的,所以问题应该以一般现在时的时态提问.

when did your brother start to work as a waiter?