When is it okay to mix banmal and jondetmal? (나 and -요)

Question

Sometimes I hear people use 나(는) (which should be banmal, right?) but then use a -요 ending (which should be jondetmal) in the same sentence. Is it really okay to mix the two? If so, what are the rules for this? Everything I read in grammar books just explains one or the other, not both of them mixed together in spoken conversation.

ETA: I'm seeing it in Duolingo, too (with no explanation). It will mix (in the same sentence) the pronoun 나 with -습니다:

나를 놀리지 마십시오.

나는 친구를 너무 믿습니다.

My understanding is that 나 is informal. So I find it odd for it to be mixed with a formal ending.

Answer 1

All endings determine the speech level of the sentence. '-요' ending change 반말 to 존댓말. As you thought, it is usually awkward to say both '나' and '-요' in one sentence. It isn't used in a formal situation or for a boss. However, there are a few times to say so.

For example, "나나 잘 할게요." is 존댓말 but it is an expression that lowers myself. It is used in situations of self-deprecation or satire.

Another is "나는 건성 피부라서 가습기를 쓰는데, (저는) 이 제품이 마음에 들어요." In this sentence, the front of comma is 반말 but the back of comma is 존댓말. This means that the former is more personal statement than the latter and the latter is what I want to say. Of course, "저는 건성 피부라서 가습기를 쓰는데, 이 제품이 마음에 들어요." is more formal.

Similarly, "나는 가습기를 쓰는데요." can be interpreted like "내 얘기인데, (제가) 가습기를 쓰는데요."

Answer 2

No official rules and standard terms describe such mixing, but I would like to say that it is okay only when the listener thinks it is.

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인칭 범주에 기반한 한국어 경어법 교육 연구 says the following:

... 청자가 상위자인 경우를 예로 들어 보기로 한다.

가. *내가 할게.

나. *제가 할게.

다. *내가 할게요.

라. 제가 할게요.

... 청자가 상위자인 경우 (가, 나)처럼 화계가 해요체나 하십시오체가 아니면 비문이 되고, (다)처럼 청자 높임의 해요체가 사용되었어도 청자가 상위자이고 화자가 스스로 겸손의 의미를 나타내고자 할 때 ‘저’를 사용하지 않으면 비문이 된다. (다)의 경우는 동급 관계에서 상호 존중의 의미로 사용하거나 상하 위계 관계에서 상급자가 하급자를 예우해줄 때 가능하기는 하지만 화자의 겸손을 나타내지는 못한다. 그러나 (라)와 같이 ‘저’가 해요체나 하십시오체와 함께 사용되면 화자의 겸손 표현이 가능하다.

According to this passage, "나 ...요." could be acceptable in these cases:

1. 동급 관계에서 상호 존중할 때 (when those at the same level respect each other)

2. 위계 관계에서 상급자가 하급자를 예우해 줄 때 (when one treat her/his inferior with respect)

I have used could because it may start an argument or fight. Because of the pronoun, 나 (which does not show one's humility), the listener could think that the speaker is rude. One of Usin Jung's examples, "나는 가습기를 쓰는데요.", may sound rude, too. It depends on the listener's judgment.

When is it usually okay? In my opinion, that is when a person says something to her/his beloved, very close one. Nonetheless, some people including me just use complete 반말 and total 존댓말. As for couples, the Korean comments here and there show the reality: There is no consensus about that usage.

Usin Jung also mentioned "나나 잘할게요" and it reminds me of a catchphrase, "너나 잘하세요." A humble person would not make these sarcastic remarks.