to learn ,
make复合结构中,不定式作宾补,必须省略to,被动中,主补时,必须加上to.例如：
His father makes him study at least 12 hours a day.不能说to study,to须省略.
被动：He is made to study at least 12 hours a day (by his father).to不能省,必须加出来.
这是固定的,希望记住,