John will tell you later
You will be told (by John) later (if you want to reveal who will be telling him/her - use it!)
Everybody loves Mr Brown
Mr Brown is loved (by everybody) (I think it would be a good idea to use it here - it is not the same to be loved by someone than by everybody...)
Somebody must have taken my wallet
My wallet must have been taken (somebody? who is somebody in this sentences? is it everybody and nobody at the same time?)
The traffic might have delayed Jimmy
Jimmy might have been delayed by the traffic (We think it's a good excuse... so say it!)
A workman will repair the computer tomorrow
The computer will be repaired tomorrow (avoid! :)
My grandfather built this house in 1943
This house was built (by my grandfather) in 1943 (you can write the agent if you want people to know it was your grandfather who built it!)
James might cook dinner
Dinner might be cooked by James. (so... he's doing all the work and you don't want to name him?)
The wolf ate the princess
The princess was eaten by the wolf (you could not use it ... but then people would wonder if it was a cannibal)
People speak Portuguese in Brazil
Portuguese is spoken in Brazil (avoid!)
They are building a new stadium near the station
A new stadium is being built near the station (unnecessary)
At six o'clock someone was telling a story
At six o'clock a story was being told (The agent wouldn't give you more information)
The cleaner has cleaned the office
The office has been cleaned (Avoid it: cleaners are usually the people in charge of cleaning the offices)
By next year the students will have studied the passive
By next year the passive will have been studied (we wouldn't use it... students study.. so it's obvious..)
Somebody should do the work
The work should be done (Somebody is not identifying anyone - so you can avoid it)
The Government is planning a new road near my house
A new road is being planned near my house (avoid agent: roads are planned by government)
Somebody has drunk all the milk!
All the milk has been drunk! (agent is omitted - somebody won't give you any further information)
By this time tomorrow we will have signed the deal
By this time tomorrow the deal will have been signed (by us) (optional: do you want to say it?)
Picasso was painting Guernica at that time
Guernica was being painted (by Picasso) at that time (I would use the agent: IT'S PICASSO!)
He had written three books before 1867
Three books had been written before 1867 (avoid it: "by him" won't add much information - you are not identifying him) (image shows Isaac Asimov)
I had cleaned all the windows before the storm
All the windows had been cleaned (by me) before the storm (it depends if you want to reveal who did the hard work.... I would haha)