Which is correct

Were you too stupid to grasp it?

Were you to stupid to grasp it?

Were you too stupid to grasp it? Is correct. Too is quantitative

Were you too stupid to grasp it?

Explanation:

To is a preposition with several meanings, including "toward" and "until." Too is an adverb that can mean "excessively" or "also." like "I want to go too!" Just to be clear: two is pronounced the same as to and too, but it can't be used instead of either of them because it's a number.