Put the transitive verbs into the passive voice. Do not mention the agent unless it seems necessary.

20 You need not type this letter.
21 This used to be number 13, but now I see that someone has crossed out '13' and written '12 A' prime underneath.
22 You mustn't move this man; he is too ill. You'll have to leave him here.
23 They searched his house and found a number of stolen articles.
24 Nobody has used this room for ages.
28 They have taken down the For Sale notice, so I suppose they have be sold the house.
29 Someone broke into his house and stole a lot of his things.
31 A lorry knocked him down.
32 They returned my keys to me; someone had picked them up in the street.
33 We had to give the books back; they did not allow us to take them home.
34 You shouldn't leave these documents on the desk. You should lock them up.
35 They handed round coffee and biscuits. 
36 They have tried other people's schemes. Why have they never tried my scheme?