-(으)ㄹ 테니(까)

In this lesson, we are taking a look at the structure -(으)ㄹ 테니(까). This is used often in situations where one person wants, or asks, the other person to do something. When you use it after a verb stem that ends with a consonant, you add -을 테니(까), and when the verb stem ends with a vowel, you add -ㄹ 테니(까). The original form is -을 테니까 or -ㄹ 테니까, but it is always interchangeable with -을 테니 or -ㄹ 테니, with the last letter, 까, omitted.


Original meaning 
-테니(까) comes from the combination of 터 and -이니(까). 터 means a “place”, “site”, “lot”, “ground”, or “foundation”. There are mainly two usages for this structure: 1. When you are offering to do something and you ask the listener to do something else in return. What you ask of the other person doesn’t always have to be a favor that you want; it can also be something that you want them to do for their own good. Therefore, -(으)ㄹ 테니(까) is often translated as “I will do this, so in return, I want you to do this”. As a result, you can not use this structure for all situations where you say “therefore” or “so”.


Example 
이건 제가 할 테니까, 걱정하지 말고 쉬세요. = I will do this, so don’t worry and get some rest. In the sentence above, you are offering to take care of something and telling the other person to go get some rest. You cannot use the same structure about someone else doing the same thing. For example, “이건 석진 씨가 할 테니까, 걱정하지 말고 쉬세요” is not a natural sentence. If you want to say something like “Seokjin will take care of this, so don’t worry” in Korean, you can either just use the -을 거니까 ending and say “진 씨가 할 거니까”, or you can keep yourself as the subject of the sentence by saying “이건 석진 씨한테 하라고 할 테니까”, meaning “I will ask Seokjin to do this, so…”. 2. When you are making an assumption and are almost certain about something, you can use this structure to mean “I assume/think/believe that this will happen/is happening, so let’s do this/please do this.” Even when you are talking about a present state or action, since you’re assuming and WILL have to check to see if you’re right, it’s always in the future tense.

Example 

밖에 추울 테니까 나가지 마세요. = It must be cold outside, so don’t go out. If you are NOT making an assumption and just saying a plainly known fact, you can use the -( 으)니까 structure and just say “밖에 추우니까”. The -(으)ㄹ 테니(까) structure is often used together with -(으)면, which means “if”. For example, if you say “지금 밖에 나가면 추울 테니까”, it means “if you go outside now, it will be cold, so…” 

Sample Sentences 

1. 제가 점심 살 테니까, 경화 씨가 커피 사세요. = I will buy lunch, so Kyung-hwa, you buy the coffee. 

2. 저는 먼저 갈 테니까, 나중에 오세요. = I will go first, so you come later.

3. 나머지는 제가 할 테니까, 먼저 퇴근하세요. = I’ll do the rest, so please go home first.

4. 저는 뛰어갈 테니까, 효진 씨는 택시 타고 오세요. = I’ll run, so Hyojin, you take a taxi. 

5. 갑자기 찾아가면 놀랄 테니까, 전화를 할까요? = If we visit her suddenly, she’ll be surprised, so shall we call her? 

6. 지금 출발하면 너무 일찍 도착할 테니까, 10분 뒤에 출발해요. = If you leave now, you’ll get there too early, so leave in 10 minutes. 

7. 아직 뜨거울 테니까 조심하세요. = It must be still hot, so be careful.
From http://www.talktomeinkorean.com/lessons/l9l16/
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~(으)ㄹ 테니까:

• Equivalent to "since it might..." or "since he/she might..."
• Gives a suggestion to the listener
• A combination of ~(으)ㄹ 터이다 (gives an intention) and ~(으)니까 (expresses a reason)
• 1st clause: provides a reason for...
• 2nd clause: ...the suggestion or advice

Usage:

• Can also be used at the end of a sentence (~(으)ㄹ 테니까요)
• Only for first person subjects
• Only verbs can precede it, but no past tense verbs
• Difference: ~(으)ㄹ 텐데 = an actual situation for giving an assumption
• Difference: ~(으)ㄹ 테니까 = the reason for giving an assumption (future tense, assumed)
• Difference: 때문에 = the actual reason for something (past/present tense, actual)
• CANNOT be used with 걱정이다, 고맙다, 미안하다

Tenses:

Adj/Verbs:

• 과거: ~았/었을 테니까
• 현제/미래: ~(으)ㄹ 테니까

Noun이다:

• 과거: 였을/이었을 테니까
• 현재/미래: 일 테니까

Examples:

과거:

• A: 초콜렛을 많이 먹으면 살 찔테니까! (You might get fat if you eat lots of chocolate!)
• V: 나쁜 습관을 만들었을 테니까 삶은 지금 부터 더 나빠질 거예요. (Since it looks like you made a bad habit, your life will get worse from now.)
• N: 장차 배우가 되려는 사람이었을 테니까 배우학교에 다녀세요. (Since he's a guy who wants to be a future actor, he ought to go to acting school.)
• A: 그 책상이 고장났을 테니까 깍까 주세요. (Since the desk looks like it was broken, you should give me a discount.)
• V: 그 영화가 좋았을 테니까 우리도 보자! (Since that movie looks like it was good, we should watch it too!)
• N: 한국의 전자 제품이었을 테니까 든든해요. (Since that's a Korean electronic device, it's probably strong.)

현재:

• A: 비 올 테니까 우리 집으로 가겠습니다. (Since it looks like rain, we should go home.)
• V: YB 지금 제일 유명한 노래를 불을 테니까 우리 같이 부르자. (Since YB is now singing their most famous song, you should sing along.)
• A: 랏데가 쓸 테니까 스럽 조금 넣세요. (Since the latte might be bitter, you should add syrup.)
• A: 대통령은 바쁠 테니까 예약을 해야 해요. (Since the President might be busy, you should make a reservation.)
• V: 운동을 열심히 하기 데문에 피곤할 테니까 꼭 일찍 자야 해. (You're probably tired from all the exercise, so you should go to bed early.)
• N: 이 것은 그 회사의 전화번호일 테니까 연락 해 주세요. (Since this is probably the company's phone number, you should call it.)

**These are notes from Korean Grammar in Use: Intermediate.

From http://keytokorean.com/classes/intermediate/intermediate-grammar-4-%EC%9C%BC%E3%84%B9-%ED%85%8C%EB%8B%88%EA%B9%8C-since-it-might-you-should/