22.3.
2 My younger sister can stay alone now, but she could not do it last year.
3 My best friend can play tennis now, but he(she) could not do it a month ago.
4 Your dog can open the door itself now, but it could not do it last week.
5 I can sing very well now, but I could not do it when I was a child.
6 He can cross the river now, but he could not do it when he was younger.
7 She can run very fast now, but she could not do it two years ago.