I know there are some nouns in Korean that have honorific forms. For example, the honorific form of 밥 is 진지, the honorific form of 집 is 댁...
So do I always have to use honorific nouns with superiors?
For example, if I ask my father "아버지, 밥을 드시고 계세요? (Dad, are you eating rice?"), is that okay?
Or do I always have to ask "아버지, 진지를 드시고 계세요?"?
Similarly, if I ask my teacher "선생님,집이 어디세요? (Teacher, where is your house?", is that okay?
Or do I always have to ask "선생님, 댁이 어디세요?"?
The simple answer is yes, you should use honorifics for your superiors, but it also depends on the particular words.
댁 (집), 분 (사람), 주무시다 (자다), 드시다/잡수다 (먹다), 편치 않다/편찮다 (아프다), 돌아가시다 (죽다) are all commonly used honorific phrases.
So 선생님, 댁이 어디세요? is a good sentence. 집이 어디세요? doesn't sound right when talking to anyone outside of your close circle.
진지 is questionable, though. It sounds old-fashioned and stilted, and it can give the impression of treating the person as old and feeble. It doesn't seem to be used much these days. You might use it with your parents, but saying something like 진지 잡수셨어요? at workplace would sound weird and comical. 식사 (eating, meal) and 식사하다 (to eat meal) are convenient all-around phrases, especially in social settings.
So 아버지, (지금) 식사하고 계세요? sounds better to me than 밥(을) 드시고 계세요? or 진지 드시고 계세요?. Speaking of 밥, it can refer to a "meal" in general or "(bowl of) rice". 밥 in the former sense and related words like 아침밥, 저녁밥, etc. also sound a little outdated because not everyone eats rice for a meal. It is safer just to say the type of meal, like 아침 잡수셨어요? / 아침 식사 하셨어요? (Did you have breakfast?), 점심 먹으러 갑시다 (Let's go to lunch), 저는 저녁은 많이 안 먹어요 (I don't eat a lot at dinner), etc. 밥먹다 (informal version of 식사하다) is widely used but it's not honorific and so inappropriate for superiors. And 밥 in the sense of rice as a kind of food can be used with anyone, as in 밥을 드시겠어요, (아니면) 다른 걸 드시겠어요?
