Answer:
1) The little boy has been found who was missing since yesterday.
2) The man who discovered a new planet has won a Nobel prize.
3) We should ring the police. ( i am not sure if this one is  correct)
4)The house is empty now where I used to live.
5) If I try to run fast I get out of breath.
6) We couldn’t get the tickets in spite of queuing for an  hour.
7)  Trevor didn’t notice the sign even though it was  right in front of him.
8) I saw Emma just now. She said her tooth is still aching.
9) I saw David yesterday. He said he was going to watch a movie.
10) I am busy in the morning on Friday.
11) You must congratulate Michelle on her success. ( i am not sure if this one is  correct)
12) Cooking, which is the passion of Robin, has made him young  chef of the year.
Explanation: