动词不定式省略to的情况：
（1）不定式在感观动词（see,notice,look at,listen to,hear,watch,feel,observe）、使役动词（let,make,have）后作宾语补足语时,通常省去to.如：
I have my students come early.
（2）不定式在help后作宾补时,可带to,也可省略to.如：
I helped him（to）find his lost watch.
注意：以上两种情况变成被动语态时要加上to.如：
She was seen to go into the classroom.
（3）如果两个或多个不定式并列使用时,常在第一个不定式前面加to,后面一般都省略.如：
To try and fail is better than not to try at all.
6.“疑问词＋不定式”组成的不定式短语在句中可以作主语、宾语和表语.如：
She told me when to start.