Is "Did you not tell me..." proper English, as opposed to "Didn't you tell me?"
Did you not tell me/Didn't you tell me?
Grammatically, both of the sentences are correct.
The first negative question, which is uncontracted type, is usually used in formal English. On the other hand, the second negative question, which is contracted type, is very much common in both spoken and written English.
They are almost similar in meaning,
In both questions you are placing the auxiliary verb did (which puts the sentence in the past tense) BEFORE the actual subject/noun,
- Did you not = Ok.
- Didn't you = The contraction between Did + Not still doesn't
change the Verb-before-Subject/Noun rule, so it should be
acceptable, although the previous sentence is definitely more
Perhaps "Didn't you?" would be more appropriate in Tag Questions, e.g., "You did tell me to do that, didn't you?" or "You haven't done it, have you?".
Consider the following:
Did you not tell me to eat the apple?
Didn't you tell me to eat the apple?
Did not you tell me to eat the apple?
The first two sentences are proper and equivalent. The third form is not a currently used colloquial interrogative form. As a declarative sentence:
I did not tell you to eat the apple.
The phrase "did not" can be used and is colloquial.