Now let’s learn about two grammar constructs that express anticipation/belief/expectation about something. (ㄹ/을) 테니까 is a conjunction and (ㄹ/을) 텐데요 is an ending.
V + (ㄹ/을)테니까
1. (ㄹ/을)테니까 is used when you are offering to do something and expect something in return.
제가 할 테니까, 걱정하지 말고 쉬세요 – I will do it, so don’t worry and get some rest.
2. (ㄹ/을)테니까 is used when you are certain that something will happen, and telling someone to do something in response. It is similar to the expression ‘Since…’ or ‘I expect…’.
밖에 추울 테니까 나가지 마세요. – Since it will be cold outside, don’t go out.
V + (ㄹ/을) 텐데요
3. (ㄹ/을) 텐데요 is the equivalent of saying ‘it must be…’, ‘it would be…’ or ‘I imagine/believe/think it would be…’
아 맞아요, 오늘이 친구의 생일일텐데요. – Oh right, it must be my friends birthday today.
여친이 있으면 좋을텐데요. – It would be nice to have a girlfriend.