For how long should I walk?What can I help you with?

这个没有什么为什么,就是这么说话的.
I help you with something,而不是I help you something.
所以问的时候就是What can I help you with?
第二个 For how long should I walk?其实和How long should I walk for?是一样的,都可以.
I walk for an hour,而不是I walk an hour.
所以问句就应该是How long should I walk for?把介词放到句首也可以For how long should I walk

for +时间，表示时间段，For how long should I walk? 我应该散步多长时间？
What can I help you with? 陈述语气是这样 I can help you with what，我可以帮您做些什么，因为后面要加名词所以要加介词with，如果是Can I help you?就不用加了。
都是语法上面的性质，不加不完整...