一. 形式
① 带to的不定式（to do sth , to be done , to be doing sth , to have done sth, to have been done ）
② 不定式的否定式 not to do
二．作用
（一）作主语
To know something about English is one thing and to master English is quite another.
To lose heart means failure.
不定式做主语时常用it作形式主语
