Passive in to Active English grammar rules

If you want to chang a passive-voice sentence to active voice ,find the agent in a "by the..." phrase, or consider carefully who or what is performing the action expressed in the verb.Make that agent the subject of the sentence,and change the verb accordingly .sometimes you will need to infer the agent from the surrounding sentences wich provide context.
Passive voice

the book _{is being read by} most of the class.
       ↓    →    →    →    ↑

Results will be published in the next issue of the journal.
     ↓      →_by?

 
Mistakes were made.
     ↓   →_by?

Passive in to Active English grammar rules

"Changed to Active voice"	Agent
Most of the class is reading the book	Most of the class
The researchers will publish their results in the next issue of the journal.	Agent not specified;most likely agents such as "the researchers"
We made mistakes.	Agent not specified ;most likely agents such as 'we'