Проверьте предложения если есть ошибки исправьте и скажите в чем ошибка . they don't understand russian language . i fall with my bike . he give me pen . my cat fast grow . i make my homework . we find little kitten. i fly in sleep . i never don't steal 15

They don't understand THE Russian language.
I FELL OFF my bike.
He GAVE me A pen.
I HAVE DONE my homework.
We have found A little kitten.
I never steal.
I fly IN A DREAM.
My cat IS GROWING FAST.