楼上答案不完全妥当.
the后面没有加单复数的严格要求.
举例：The students are singing.
只有the后面家集合类名词,形容词,单数名词,或者是独一无二的东西才会只能用单数.
举例：the old,the rich,the sun,the moon
你提问的the flowers没问题,举例：the flowers in the garden are beautiful.