begins class和classes begin为什么加s?

class是单数时,begin就要加s.class是复数,也就是classes时,begin就不用加s了.
class是名词,begin是动词.
名词单数时,单词不加s或者es；如果名词是复数时,后面就要有s或者es.
而动词前面的名词是单数时,动词后加s或者es；动词前面的名词是复数时,动词就不用加s或者es.
名词单复数怎么区别?简单地说就比方说：一只狗是单数,两只以上就是复数.

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