如句子的主词含有不定词词组 (for + NP) + to V，常用it代替，再把不定词词组移到后面。
To work with you is nice. = It is nice to work with you.
To study English every day is important. 每天读英文很重要。
= It is important to study English every day.
To learn two foreign languages at the same time is difficult.
= It is difficult to learn two foreign languages at the same time.
To make a reservation in advance is necessary.事先订位是有必要的。
= It is necessary to make a reservation in advance.
To get him to change his mind is not easy. 要让他改变心意不容易。
= It is not easy to get him to change his mind.
To tip waiters is customary in America.
It is customary to tip waiters in America.