Модальные глаголы.
1.I didn't understand that chemistry lesson at all!
I you if you had asked me!
2.I did so badly in my speaking test!
Well, you more then!
3. I wonder why Tom looks so happy today. He just first prize in the school art competition!

1. I didn't understand that chemistry lesson at all!

I would have explained it to you if you had asked me!

2. I did so badly in my speaking test!

Well, you should have practiced more then!

3. I wonder why Tom looks so happy today.

He must have just got first prize in the school art competition!