Английский язык.Нужно составить предложения с саn и could

My younger sister can stay alone now, but she could not do it last year.My best friend can play tennis now, but he could not do it a month ago. Your dog can open the door itself now, but it could not do it last week. I can sing very well now, but I could not when I was a child. He can to cross the river now, but he could not when he was younger. She can run very fast now, but she could not two years ago.