1. 如果表语是不定式，主语也是不定式；表语是动名词，主语也是动名词。如：
To see is to believe.＝Seeing is believing.
2. 动名词作主语的句型。如：
It is no use/no good/useless doing sth.
It is a waste of time doing(也可用It is a waste of time to do)
3. 动词不定式和动名词的复合结构：
动词不定式的复合结构有两种：It is difficult/easy/possible/necessary/...for sb. to do sth.和It is kind/wise/foolish/considerate/...of sb. to do
动名词的复合结构是由名词所有格或“物主代词＋动名词”构成。在口语和非正式语体中，只要不是作主语，动名词复合结构中的逻辑主语常采用名词通格或人称代词宾格。如：
His coming made me happy.
I can't imagine his/him living there alone.
4.动名词被动式作主语(过去分词不可作主语)。如：
Being exposed to the sunlight for a long time does harm to your skin.
