1.动名词短语、不定式短语、名词性从句做主语，谓语用单数。如：

Buying clothes is often a time-consuming job because those clothes that a person likes are rarely the cones that fit him or her.

(1987年考研题)

To understand the situation completely requires more thought than has been given thus far.

2.表示时间、距离、金额、重量、面积、体积、容积等度量的名词短语做主语时，谓语用单数。

Two weeks was too long

Five times five makes twenty five

3.一般用and连接的两个单词或短语做主语时候，谓语用复数，但是下面用and连接的主语表示一个概念，谓语用单数：

law and order 法制 soap and water 肥皂水

a cup and saucer 茶杯碟子 fork and knife 刀叉

the needle and thread 针线 trial and error 反复尝试，不断摸索

horse and carriage 马车 time and tide 岁月

bread and butter 奶油面包 the ebb and flow 盛衰，潮涨潮落

如： If law and order not preserved， neither the citizen nor his property is safe.

A. is B. are C. was d. were

答案：A.

4.表示学科和某些疾病名称的名词是复数形式，作主语时候谓语动词用单数形式

Linguistics is a branch of study on human language.