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Sentence 1: I didn't think it was possible
Sentence 2: I had no idea it was possible
A friend asked me if Sentence 1 was grammatically correct or not. I advised him to rewrite it as Sentence 2 in case Sentence 1 didn't sound right to him. But he didn't agree to it because he thinks 'did' is used when something has been performed and 'had' is used to refer to something that someone possessed
There is a subtle difference between the two sentences, however they are closely related:
"Have" here indeed refers to possession. The person didn't hold any idea in his head - it wasn't there. "Do" here refers to performance but it's an auxiliary verb here in the negative form. The main verb "to think" refers to an action.
Both sentences are grammatically correct; however, at least to me, the difference in meaning is all but subtle. Without any further context, Sentence 1 merely states what the speaker thought was possible, whereas sentence 2 clearly conveys great surprise or even incredulity.
The discussion about the function of 'did' and 'had' has no meaning here, since there first is an auxiliary verb and the second is the main verb. You are comparing the proverbial oranges and apples.
He is right that did is used when something is performed and had is used when something is possessed. The second sentence is in fact correct because the idea is being possessed here. In other words, someone had no idea that it was possible.
In my opinion, both are grammatically correct, but, with a significant difference. The first sentence indicates an effort was made by the speaker to evaluate the possibility and through that evaluation had determined it to not be possibile. The second sentence indicates that the speaker had no forethought of even the existance of a possibility. Thus, never thought about it at all.
I would argue that "thinking" is definitely "performing" ... as much as "having a headache" is possessing! Both sentences, however, are incorrect! Neither has a period or exclamation mark at the end!
