-(으)ㄹ 테니까

-(으)ㄹ 테니까 gives the speaker's intention ('I will…') or strong guess ('it will probably…') as the reason for a request or suggestion to the listener.

이유·원인Reason & cause

Han: -(으)ㄹ 테니까 normally leads into advice, a request or a suggestion, not a report of a past action; for a past situation say 어려울 것 같아서 공부했어요.

Examples

이 책은 내일 꼭 돌려줄 테니까 오늘만 빌려 줘.I'll definitely give this book back tomorrow, so lend it to me just for today.

제가 설거지할 테니까 좀 쉬세요.I'll do the dishes, so please get some rest.

지금쯤 회의가 끝났을 테니까 전화해 보세요.The meeting has probably finished by now, so try calling.

주말에는 사람이 많을 테니까 평일에 갑시다.There will probably be a lot of people on the weekend, so let's go on a weekday.

V/A+(으)ㄹ 테니까 (past guess: V/A + 았/었을 테니까)

가다 → 갈 테니까, 먹다 → 먹을 테니까, 끝나다 → 끝났을 테니까

With a first-person subject (I/we) it shows the speaker's intention; with other subjects or adjectives it shows the speaker's guess.
The second clause is usually a command, request or suggestion (-(으)세요, -(으)ㅂ시다, -자, -아/어 주세요), not a statement of what already happened.
To guess about something already finished, use -았/었을 테니까 (e.g. 도착했을 테니까).
It is not combined with -겠-: say 할 테니까, not 하겠을 테니까.

Wrong: ~~시험이 어려울 테니까 열심히 공부했어요.~~

Right: 시험이 어려울 테니까 열심히 공부하세요.

-(으)ㄹ 테니까 normally leads into advice, a request or a suggestion, not a report of a past action; for a past situation say 어려울 것 같아서 공부했어요.

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