I have problem with some gramaticaly sentences

Hi ,

I don't succeed to Rewrite without changing the meaning of the original sentences of that setneces:

1)If you want my opinion,you should stop eating so many sweets.
If I__________________________________________________

2)People say he is an excellent pianist.
He is said to be excellent pianist. am I right?

3)This is the first time I've been backstage at the theatre.(never)
____________________________________________

4)No one has signed this certificate.
This _________________________________________

5)I haven't had such a good time for years.(enjoyes)

6)I dont this it's his fault.(blame)
__________________________________________________

Thanks very much .