could you give me a hand with this box?can you give me a han

不是所有 can you 开头的问句都可有 could you 代替
could you 是 can you 的委婉语态,
因此请求问句 can you 可改用 could you,
但其他情况下不可,比如
can you walk,or would you like me to get a taxi for you?
我是加拿大人,前英语老师.