washing food down with water as a substitute for chewing is not a good idea,but,some water with meal

题目

washing food down with water as a substitute for chewing is not a good idea,but,some water with meals has been found to be helpful
1.substitute for前面为什么会有冠词?
2.to be helpful是some water with meals的主语补语?
3.to be helpful是不定式担当主补?

答案

She'll probably have a substitute,though.
虽然如此,她可能会找个代课老师来.
这里的substitute应该是做了可数意义,所以用a,然后for则是对substitute的修饰了
to be helpful是做了补语的,是不定式做了主语补足语