Ответ:
1) I have been dieting for the past few years. I have tried all sorts of diets.
2) I have been taking dancing lessons since last year. I have learned three dances.
3) I have been studying English since I was 7. Now I can speak fluently.
4) -Oh, you look great!-Yes, I have been riding my bike for a month.
5) I have been doing sports for two years. I have had good results.