因为这是不定式主动表被动的
english和动词learn是主动关系
①不定式用主动形式表被动意义的情况：
动词不定式作定语时,当不定式动作的发出者是句子主语、宾语或者说话的人时,不定式用主动形式表被动意义.如：
Can you give me a book to read?
I can’t go with you now because I have masses of work to do.
反射不定式中,即“sb/sth +be +adj/n + to do.” 结构中,不定式用主动形式表被动意义.如：
English is easy to learn.
Peter is a person to get on well with.
②动名词用主动形式表被动意义的情况：
（需要）need(require, want, demand)doing = need(require, want, demand )to be done
（值得）deserve doing =deserve to be done , be worth doing.(=be worthy to be done / be worthy of being done)
