Can all verbs be changed into nouns by adding Er after them Today, I read a word reformer revolutionist, which is not found in baidu dictionary. I can only find reformer revolution, so I think that killing is followed by Er, which is a killer. There are still many such cases. Is it possible to add Er after all verbs to change nouns, such as Sayer speaker, seeer viewer and typewriter?

Can all verbs be changed into nouns by adding Er after them Today, I read a word reformer revolutionist, which is not found in baidu dictionary. I can only find reformer revolution, so I think that killing is followed by Er, which is a killer. There are still many such cases. Is it possible to add Er after all verbs to change nouns, such as Sayer speaker, seeer viewer and typewriter?

Well, not all of them, but most verbs that can be changed into nouns end with ER
But the examples you give are not very good, such as Sayer speaker, but people usually don't use Sayer, so it's wrong to use viewer or seeker
But there are many other examples, such as act performance, actor / act; music is music, and musicians are musicians
Therefore, not all verbs can become nouns, and those that can become nouns don't necessarily end with ER (most of them end with ER)
When you read more and more English, you will have a summary of your own, knowing that the common ones are wrong