1. 作主语

    作主语的不定式如果很短，通常位于句首，如：To see is to believe.若不定式（短语）较长时，常用it作形式主语，而把真正的主语（即动词不定式）放在后面，以避免头重脚轻。常见句型为：“It”s +名词（for + sb）+ to do sth“和”It“s +形容词（for / of + sb）+ to do sth”。注意：在这个句型结构中，若形容词表示与人有关的“赞美”或“批评”时用of，如kind，nice，good，clever，bad等；否则用for.如：

    It“s easy for you to answer the question.

    It“s very kind of you to help me.

    2. 作表语

    动词不定式可以作be，seem，appear等系动词的表语。如：

    Her job is to take care of these children.

    3. 作状语

    动词不定式放在come，go，stop等动词后作目的状语；放在sorry，glad，surprised，pleased，angry等形容词后作原因状语；动词不定式作状语表示结果时常与副词too或enough连用。注意：too…to与so…that…和（not）…enough to…可以相互转换：如：

    She is so weak that she can“t move the box.

    =She is too weak to move the box.

    =She is not strong enough to move the box.

    4. 疑问词+不定式

    疑问词+不定式=名词短语，可作主语、宾语或表语。注意：名词短语可以由名词从句转换而来；疑问代词what，which，whom可以作宾语，疑问副词how，when，where，whether不能作宾语。如：

    I didn“t know what I should do. = I didn”t know wheat to do.

    I didn“t know how I should do it. = I didn”t know how to do it.

    5. 几种不带to的动词不定式句式

    had better…， Why not…？ Will / Would you please…？等句型后接不带to的不定式。如：

    You had better stay at home.

    Why not see a doctor？