- 먹느냐? -- Do you eat? / Are you eating?
- 먹었느냐? -- Did you eat?
- 먹겠느냐? -- Are you going to eat?
- 연필이 있느냐? -- Do you have a pencil?
- 연필이 없느냐? -- Don't you have a pencil?
- 크냐? -- Is it big?
- 작으냐? -- Is it small?
- 책이냐? -- Is it a book?
- 모자냐? -- Is it a hat?
There is also the reflective past tense ending 더냐, which is used when the listener is asked about a personal experience or observation he had in the past . It can be used with both adjectives and verbs. See the following examples:
- 잘 있더냐? -- Have you been well?
- 그 어떻더냐? -- How was it?
- 둥글더냐? -- Was it round?
- 모나더냐? -- Was it angular?
- 무엇이더냐? -- What was it?
I think it used to be possible to abbreviate 더냐 to 냐, but that no longer seems to be the case.
Today, many younger Koreans are using 냐 as an abbreviation for 느냐, but such an abbreviation is not recognized by Korean dictionaries. Therefore, 먹냐 and 먹느냐 are both being used to mean, "Are you eating" or "Do you eat," but 먹냐 is considered an incorrect form since 냐 is supposed to be attached only to adjectives.
HERE is more on 느냐/냐 from 남영신, who is another guy I respect and enjoy reading. He mentions that the 느냐/냐 ending is used a lot in the Jeolla region while the 나 ending is used in the Gyeongsang region. However, according to 이수열, the 느냐/냐 ending and the 나 ending are not really equivalent since 느냐/냐 is considered 낮춤 말 while 나 is considered 반말. In other words, you cannot make the 느냐/냐 ending polite by adding anything, but you can make the 나 ending polite by simply adding "요" (e.g. 벌써 집에 가나요?). 반말 (half speech) is just polite speech with the polite half (i.e. "요") removed. 남영신 also says the 니 ending is used in the Seoul/Gyeonggi region and that it is one of the first things about the Seoul dialect that Koreans learn when they migrate to Seoul.