Our teacher did what she could--(help)us with English 一直以为填t

Our teacher did what she could__(help)us with English 一直以为填to help .老师却说填原型,说是和动词
有关,就是前面的did,还举了个例子 I did nothing but do.老师说did如果改成其他什么词就填不定式.
看见网上其他回答都说是to help 很迷茫