名词性从句：从句在主句中的成分相当于名词，又被称为名词从句；
根据从句在主句中所做的成分，名词性从句可以分为以下4种：
例如：
（1）That the seas are being overfished has been known for years.
海洋正在被人们过渡捕捞，许多年来这已是尽人皆知的事情了。
（2）I have learned that love, not time, heals all wounds.
我明白了是爱，而不是时间能治愈一切创伤。
（3）The reason for my return is that I left my keys behind.
我又回来，是因为我忘带钥匙了。
（4）An idea came to her that she might do the experiment in another way.
她突然想到，她可以换个方法做实验。