(3)作宾语：如果不定式（宾语）后面有宾语补足语，则用it作形式宾语，真正的宾语（不定式）后置，放在宾语补足语后面
Marx found it important to study the situation in Russia. We think it important to study hard.
动词不定式也可充当介词宾语，如： I have no choice but to stay here. He did nothing last Sunday but repair his bike.
动词不定式前有时可与疑问词连用，如：He gave us some advice on how to learn English. He asked me what to do next.
