To understand the different interrogative sentences, you need to know more about the sentence ending forms. There are plain (no special nuance) endings in use in everyday normal conversation - only four are in wide use now. These four (I'll call them dialog ending forms) only differ in politeness and formality. -아/어요 in your list is one of these dialog endings (it's the most frequently used on among adults), while the others are special endings outside of this group. Here are the dialog endings (for 가다 and 이다 in present tense), in statement and question forms. 1. -ㅂ니다 (formal, polite) => (학교에) 갑니다, (학생)입니다 / (학교에) 갑니까? (학생)입니까? 2. -다 (plain, impersonal) => 간다, (학생)이다 / (학교에) 가냐?/가니? (학생)이냐?/이니? 3. -아/어요, -예요/이에요 => (ordinary, polite), 가요, (학생)이에요 / (학교에) 가요? (학생)이에요? 4. -야/어, -(이)야 => (familiar) 가, (학생)이야 / (학교에) 가? (학생)이야? (For #3 and #4, the statement and questions are the same - tey only differ in intonation) -나요?, -ㄹ까요, -ㄴ가요 are special in that they typically ask a general, objective question, about existing fact or possibilities and not personal matters (나요? might get slightly personal sometimes). They can also ask a self-directed question, like "I wonder ..." in English. All in all, they sound more cautious and polite, and less intrusive. The dialog questions are usually straightforwardly directed to the other person and very often related to the person. Here are personal question examples. -나요? -ㄹ까요?, etc are unnatural with these. - 어떤 일(을) 하세요? What kind of work do you do? (하세요 is an honorific version of 해요) - 휴가 때 뭐 할 거예요? What are you going to do during vacation? And here are the other kinds. - 사무실은 몇 시에 문을 여나요? What time is the office opening its door? - 저 사람은 왜 저럴까? I wonder why he's acting like that. (self-directed) - 여기가 동대문으로 가는 길인가요? Is this the way to 동대문? Note that there are also other question endings beyond the two types mentioned.