In English, you can only answer yes / no questions with variations on the themes of yes, no, and I don't know. French, however, has another possibility: yes in response to no.
Knowing how to ask questions in French is essential for making plans, shopping, traveling, getting to know people, and any other activity that requires obtaining information. There are two different types of questions, and different ways to ask each type.
Obviousness, knowledge, certainty, and hope are considered real - at least to the speaker - and therefore do not call for the subjunctive. Using these verbs and expressions in the negative or interrogative, however, indicates doubt: thus the subjunctive is required.