Add about my first answer... First, by default, when you creating a interrogative sentence in Korean, the question mark is added at last of the sentence. And, '-요' form is more polite significance at the Korean interrogative sentence. One thing during very big difference between Korean and English is that. The Korean sentence has levels of honor, similar politeness. For example, I'll explain. Here is the sentence. <"그는 그곳에 있나요?"> It means, "Is he in there?" When you are in 'situation (1)' (situation (1) : you are communication with somebody and you and somebody are equal status), you can use this form of sentence, "그는 그곳에 있니?" or "그는 그곳에 있어?" When you are in 'situation (2)' (situation (2) : more polite situation than 'situation (1)', for example, when you ask to somebody and somebody is older than you or have higher status than you), you should use this form of sentence, "그는 그곳에 있나요?" or "그는 그곳에 있어요?" In 'situation (2)', these two sentences are same levels. When you are in 'situation (3)' (situation (3) : when you ask somebody, and the somebody has very higher status or the communication at the best high situation, for example, you ask to your CEO or boss of your company, etc...), you use this form of sentence, "그는 그곳에 있습니까?" may be better question. Because the form of sentence "그는 그곳에 있습니까?" is more polite than "그는 그곳에 있나요?" or "그는 그곳에 있어요?" Ultimately, the sentences "그는 그곳에 있나요?" or "그는 그곳에 있어요?" and "그는 그곳에 있습니까?" have little difference. Just, degree of politeness. Your question is little grammatical question. I answered based on what I've learned. So, my answer is can not be correct. And, I don't know my answer to your question is appropriate. But, I tried effort, exceedingly. I hope my answer helps to you. Thank you.

I think -나요 is most informal among verb endings which has -요. I'd inclined to use -아/어요 instead of -나요 in general, but you could say also "뭐가 맛있을까요?" or "뭐가 맛있지요(맛있죠)?" when you feel bored with -아/어요 at times. -는가요 sounds very old-fashioned. e.g. 맛있는가요? I've heard middle-aged people said something like "어디 가시는가요?".

There are many forms about commonly used for descriptive verbs to form a question. ⇒ ('-야?', '-어?', '-니?', '-요?', '-어요?', '-나요?', '-ㅂ+니까?', '-까요?', etc) Because ('-야?', '-어?', '-니?', '-요?', '-어요?', '-나요?', '-ㅂ+니까?', '-까요?', etc) are added the end of sentence, and make the sentence, meaning of question. Just, '-어요?' is often combined with a past verb. And, '-까요?' is used at simple question, but, it is used at question about some guess. In all Korean sentences, the form which locate at last of verb are determined. According to not only the form of the verb but also how naturally the words are connected.

피곤해요.(not 비곤해요) is a polite version and 피곤해 is not. You can use -습니까? as well. It's formal but a bit unfriendly and cold.... e.x 뭐가 맛있습니까?