(1) Does he not know?
(2) Doesn't he know?
I don't usually see and hear questions formed in the first style. I was even surprised to know that it is grammatically correct. And actually it is the most "proper" way to form an interrogative sentence. Ngram shows that the first style was much more popular before than now.
What are your suggestions on choosing between these styles? And I will probably add the third one.
(3) He does not know?
EDIT:
The answers have explained how to form a question, but have not actually provided an account on the usage of the first two forms. Especially I'm interested in the first form: does it sound formal, is it used in speech or only in writing, et cetera.
Best Answer
The basic (declarative) sentence is:
You can turn this into an interrogative clause with subject-auxiliary inversion. Just switch he and does:
You can optionally replace does not with doesn't in the original sentence, using the suffix -n't rather than the word not:
Now the auxiliary is the single word doesn't, so if we apply subject-auxiliary inversion, we get:
Lastly, you can turn the original sentence into a question by using rising intonation at the end, which you indicate in writing by replacing the period
.with a question mark?:However, this is only appropriate in certain situations, for example as an echo question—repeating what you heard as a question to express incredulity or to confirm that you heard the speaker correctly. Most (but not all) situations where this would be appropriate are informal, and as a result "He does not know?" sounds somewhat unusual. Instead, you can use the contracted form:
But most of the time the interrogative form is more appropriate.