将单数句变为复数句，通常要遵循的规则是：变、删、留。
[融会贯通]
变：将单数句中的指示代词、人称代词、物主代词、名词及动词变为对应的复数形式，如this→these， that→those， he / she / it→they， I→we， my→our， his / her /
its→their， am / is →are， does→do等。
删：将单数句中的不定冠词a / an删掉。 如：That is a watch. → Those are watches.
留：保留单数句中的形容词、定冠词 the、副词、疑问词、介词等。如：
This is a nice book.→These are nice books.
What is that?→What are those?
The boy in red is my cousin.→The boys in red are our cousins.
He usually does his homework at home.
→They usually do their homework at home.
要将复数句变为单数句时，逆向进行转换即可。
