They would have used passive voice rather than active voice !
I have recently read from LIFE book, (National Geographic Learning, intermediate level) this sentence:
"....yes, but the recipe says red beans must boil for fifteen minutes or they aren't safe to eat. Do you think that's right?"
I think red beans are "things", how can they boil themselves?
So, should the sentence be corrected like this:
"....red beans must be boiled for fifteen minutes...."
Other time, I read somewhere something like : " .... the sales rise 50%...."
I dont really know why thing such as sales can rise themselves.
Thank you !
(First of all, I'm not a native speaker so some sentences have wrong grammar or vocabulary.)
I think you should know the difference between transitive verb and intransitive verb.
Intransitive verbs are the word that don't add the object.
Sit and Sleep are intransitive verb.
So the sentence with intransitive verb can not be changed to passive voice because there is no object.
Transitive verbs are the word that have to add the object to complete the meaning.
I read a book.
You play a guitar.
Read and Play are transitive verb
So the sentence above can be changed to passive voice like these,
A book is read by me.
A guitar is played by you.
Do you know 'rise' is intransitive verb so
The sales rise. = The sale is up.
But 'raise' is transitive verb
Someone raises the sales.
The sales is raised by someone.
'boil' can be both intransitive and transitive verb
Note: when you look up the meaning of verbs in the dictionary, you will see the abbreviation 'I' and 'T'.
Please remember that I = Intransitive, T = Transitive
I hope this is helpful for you. If you have any question or I'm wrong, please let me know. I want to discuss this too.