N. + ㄹ/일 텐데(요) / V. + ㄹ/을 텐데(요)

This short connective form of -을/일 터인데 indicates the speaker's expectation or anticipation. It is derived from the verb 터이다 "to expect" and the connective -ㄴ/는데.

(a)

• 동부에 단풍이 한창일 텐데(요).
  => The fall foliage must be wonderful in the East.
• 오늘이 동생 생일일 텐데 케이크를 먹었는지 모르겠다.
  => Today must be my younger sibling's birthday, but I don't know whether he/she had a cake.

 (b) 

• 피곤할 텐데 자지도 않고 책을 본다.
  => He must be tired, but he reads on without sleeping.
• 날씨가 추울 텐데 반바지를 입고 돌아다닌다.
  => The weather should be cold, but he is walking around in shorts.

Note that V. + ㄹ/을 테니(까) means "since/because (it is expected to)." It indicates the speaker's expectation in regard to an event or an action. This construction comes from the connective (으)니까 "because" attached to the ㄹ/을 터이다 "is expected" ending.

• 걸어가야 할 테니(까) 편한 신을 신으세요.
  => Since we'll have to walk, please wear comfortable shoes.
• 내일까지 다 마칠 테니(까) 너무 걱정하지 마세요.
  => I'll finish them all by tomorrow, so don't worry too much.
• 날씨가 추울 테니까 두꺼운 옷을 입고 나가세요.
  => The weather will be cold, so go out with thick (warm) clothes on.
• 버스 타면 늦을 테니까 지하철을 타자.
  => We'll be late if we take a bus, so let's go by subway.