Since it will be so: -(으)ㄹ 테니까

After a consonant the pattern is -을 테니까: 먹을 테니까, 많을 테니까. After a vowel the stem takes an l and then 테니까 as a separate word: 갈 테니까, 할 테니까; a stem in l simply adds 테니까: 만들 테니까. 테 is the bound noun 터, “expectation”, fused with 이다, so it is written apart from the verb. The short form drops the last syllable: 갈 테니.

With “I” as the subject, it offers the speaker's own plan as the basis for what the listener should do: 제가 청소할 테니까 요리는 부탁해요, “I will do the cleaning, so could you do the cooking”. It is a small deal: here is my part, now yours.

With any other subject, it offers a confident guess as the reason: 길이 막힐 테니까 지하철을 타세요, “the roads will surely be jammed, so take the underground”. For a guess about something already past, the past goes inside: 피곤했을 테니까 쉬세요, “you must have been tired, so have a rest”. The second half is usually a request, a suggestion or advice.

Next to -(으)니까, which gives a known fact as the reason, this pattern gives something that has not happened yet or that the speaker only assumes. In casual speech 갈 거니까 does a similar job. The mistake to avoid is in the spelling: 할테니까 without a space is wrong, because 테 is a word.