Is the use of 해야 하다 with 수 있다 for a descriptive verb redundant/necessary/natural?

Question

A fairly basic grammar pattern in Korean is the (으)ㄹ 수 있다 pattern that expresses the ability or possibility of a verb. Some sources I found do specifiy that descriptive verbs can be used but are light on actual examples. So anyway, while going through TTMIK’s Half in Korean & Half in English, lesson 3, I came across a curious use of the pattern:

현우: 아무튼, 아재 개그 저는 정말 좋아해요. 이게 똑똑해야 할 수 있거든요.

Cassie: That’s right. You’ve got to be very smart. We’re the only smart ones in the office, right?

My confusion stems from my brain translating the sentence partway into English and wondering why it was used in conjunction with the pattern expressing a necessity with 해야 하다:

1. (Original) 이게 똑똑해야 할 수 있거든요 For this, you have to [be able to] be clever

vs

2. 이개 똑똑해야 하거든요 For this, you have to be clever

So the second sentence is an edit by me which should have the exact same meaning. I also find expressing the first sentence literally in English sounds awkward: You do not possess the ability to be clever, either you are clever in a situation or you are not. So my question is, in practicle terms, is the addition of 수 necessary/more natural, and why?

Answer 1

You are confused by the unusual structure of the sentence: "-해야" and "-할 수 있다" are, in fact, not connected together here. I think it's easier to understand a slightly longer version:

이게 [똑똑해야 할 수 있는 일]이거든요.

Now, 똑똑해야 할 수 있는 일 can be easily understood by rearranging:

똑똑해야 (이 일을) 할 수 있다. = [You] can do (this) only if you're smart.

Hence,

똑똑해야 할 수 있는 일 = Something you can do only if you're smart.

Going back to the original sentence,

이게 똑똑해야 할 수 있거든요. = This is something you can do only if you're smart.

= You need to be smart to (be able to) do this.

In fact, if you remove "할 수 있다" part and just say "이게 똑똑해야 하거든요", then it sounds worse to me.

• Finally, when "-해야 할 수 있다" is actually connected together, it isn't "have to be able to", but the reverse order: "may need to".

  E.g., "기름이 모자라면 중간에 주유소를 들러야 할 수(도) 있다." (If we're short on gas, we may need to stop at a gas station.)

Answer 2

From my viewpoint, it is incorrect to say "이게 똑똑해야 할 수 있거든요" (This does not use the pattern 해야 하다 as the others also said). The subject 이게 should not be the object for the verb 하다 (to do) or the subject for the adjective 똑똑하다, but he used it as the object for the verb 하다. I suppose he habitually uses 이게 to start that kind of sentence without thinking about the rest. The misuse of markers occurs quite a lot in conversation.

From another point, which seems to be why you use for this, 이게 might be regarded as an interjection indicating that something is about to be said regarding this. Since no others said the term interjection here, this point does not seem to be widely accepted. In addition, dictionaries have never dealt with such use. At least, I disagree that 이게 is an interjection for those sentences.

이게 usually refers to an inanimate thing; when you use the pattern 해야 하다, it is incorrect to say "이게 똑똑해야 하거든요" unless you use the adjective 똑똑하다 to mean 또렷하고 분명하다 (Of course, 이게 can be used to disparage a person, but it does not relate to your sentence). When yours is correct, it means "This (thing) needs to be clear and distinct." You should also note that the sentence "This (thing) needs to be clever" does not usually make sense. This is because the meaning of a word like the adjective 똑똑하다 depends on whether the subject is animate.

Nevertheless, both sentences will be well understood. Using the context, people can correct and interpret what they have heard. I would just add that he should have said one of the following:

1. 이건 똑똑해야 할 수 있거든요.
2. 이게 똑똑해야 할 수 있는 일이거든요.
3. 이게 똑똑해야 할 수 있는 거거든요.

Sentence 1 has the topic marker ㄴ for the word 이거; 이건 is the object for the verb 하다. Sentences 2 and 3 connect the subject 이게 to the noun phrase/clause attached to the predicative postposition 이다.

Answer 3

That sentence actually doesn't use the pattern "해야 하다" (meaning 'have to') - it uses the ending -야 in conjunction with the verb 하다 "to do";

The clause ending (어/아)야 is used to express "you have to do the first thing for the second to be true"; e.g.:

잘 먹어야 건강을 회복한다 (you have to eat well in order to regain health).

The first thing (eating well) is necessary for the second to happen (regaining health).

So your sentence could be written like this:

똑똑해야 아재 개그 할 수 있거든요. You have to be smart in order to be able to make Ajae jokes.