楼主想问的其实就是什么情况下谓语动词用单数吧
本不想长篇地贴，不过为了你看得清楚，贴~
A. 主语为表示数目、度量、时间、金钱、距离等的复数名词，但看作整体时。
One hundred kilometers is a long distance. 100公里是一段很长的距离。
Five plus two is seven. 五加二等于七。
One hundred million dollars is a big sum of money. 一百万美元是一大笔数额的钱。
B. 主语为抽象名词、物质名词和一些习惯以单数形式表示或只有单数形式的名词，如snow, money, food, equipment, furniture, luggage, machinery, jewellery等时。
All the furniture in his house is made of wood. 他家里所有的家具都是用木材做的。
His luggage is very heavy. 他的行李很重。
C. 主语为复数形式却用作单数概念的名词，如用作学科名称的mathematics，politics等；专有名词the United States, the New York Times, the United Nations等时。
Physics is my favourite subject. 物理是我最喜欢的科目。
The United States celebrates its birth on the fourth of July. 美国7月4日庆祝国庆。
The New York Times is a popular newspaper in the city. 纽约时报是在这个城市很受欢迎的报纸。
D. 主语由某些词组如the number of, one of, each of, any one of, neither of, either of等加复数名词或代词构成时。
Each of the houses is painted a different colour. 每一座房子都漆成了一种不同的颜色。
Any one of the articles on the desk deals with air pollution. 桌上的任何一篇文章都谈及空气污染。
Either of the plans is equally impractical. 两个计划中无论哪一个都不实用。
Neither of my friends has come yet. 我两个朋友一个都没来。
注意：a number of 加复数名词作主语时，谓语动词用复数。
A number of students in this class have been to Beijing. 这个班上好些学生已去过北京。
E. 主语为many a, more than one, either, neither 加单数可数名词时。
Many a ship has been wrecked on these rocks. 许多船在这些岩石上触礁沉没。
More than one student has realized the importance of learning English. 不止一个生意识到学习英语的重要性。
Either plan is OK. 两个计划中无论哪一个都不错。
F. 以everyone, anybody, somebody, no one等作主语，或以every, each, some, any, no等修饰单数名词作主语时。
In a small village everybody knows everybody else. 小村庄里的人彼此都很熟悉。
Every cook praises his own broth. 每个厨师都夸自己的汤煮得好。
There is no one to look after the child. 没人能照看这个小孩。
G. 动名词，不定式短语，或名词性从句作主语时。
Buying clothes is often a time-consuming job. 买衣服往往是一件很费时间的事。
When they will start planting tress has not been decided yet. 他们何时开始植树尚未决定。
How he became a famous scientist is not known to us. 我们不知道他是怎样成为著名科学家的。
注意：what引导的从句作主语时，谓语动词的单、复数形式根据所表达的意思决定。
What he said is wrong. 他说的话是错的。
What he left me are only a few books. 他给我留下的只是几本书。
参考资料：http://courseware.ecnudec.com/yjs/ygg/ygg06/gg06a/gg06a4/gg06a474.htm