Почему в единственном числе в утвердительной форме пишется That is a flower, а во множественном числе Those are flowers?

That is a flower. — Those are flowers.

Plural is formed with the help of the suffix – s or –es. A flower is singular and flowers are plural. We add –s to flower and we don\'t use indefinite article (a) in plural. The plural of that is those. That’s why those is instead of that. Is is the singular form of the auxiliary verb to be. And are is the plural form of the auxiliary verb to be. That\'s why are instead of is.