What is the affirmative sentence of this negative sentence?
He left no stone unturned.
I was thinking...
Every stone was turned by him.
It doesn't seem to be the possible answer.
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The sentence "He left no stone unturned" is abnormal because it's an idiom. That means that there's really no affirmative version unless there's an affirmative idiom that means the same thing. "He turned over all stones" is the closest you'll come using the same vocabulary, but it's not an idiom even though it's almost natural and idiomatic: it'd have to be ""He turned over all the stones" to be really normal.
The essence of the idiom is, as Jim says in his comment, "He tried everything" or "He did everything he could". There are many ways of saying this, but every option fits a specific context. The idiom "He left no stone unturned" is more general and is appropriate for many different contexts, as is usually the case with clichés.
Why do you want an affirmative for this negative-affirmative? Do you have a specific context?
If you consider an alternative expression.
then there is a simple inversion to create
which sounds idiomatic as well as logical.
Arguing by analogy, we get
which "ought" to be correct even if it soumds odd.