-(으)ㄹ 테니까 is the brother of the form -(으)ㄹ 텐데. However, while they look very similar they are very different and serve a completely different purpose. The use of this form is to give the assumed reason for what is said in the second clause. Generally, that second clause is some form of advice or suggestion.
-(으)ㄹ 테니까 consists of two parts. The first part is -(으)ㄹ 테이다 and, as mentioned once before, it expresses a particular future or an intention to do something. That is then combined with the -(으)니까, which expresses reason. Combined together it creates the form -(으)ㄹ 테니까. Sentences
Sentences using this form express a suggestion or advice in the second clause. Therefore, the second clause tends to end with forms such as -(으)세요, -(으)ㄹ래요, -(으)ㄹ까요, etc. The first clause provides the reason as to why you are giving the advice. The reason is based on an assumption or supposition.
가: 요리하고 있나요? 제가 도와줄까요?
나: 농약이 아마 야채에 남아 있을 테니까 사용 전에 씻으세요.
A: Are you cooking? Shall I help you?
B: There might be some pesticide remaining on the vegetables so before we use them, please wash them.
How To Use
The conjugation of -(으)ㄹ 테니까 is similar to the conjugation of -(으)ㄹ 텐데 as you might have expected. Like its brother, it follows the basic ‘batching/no batchim’ rule. Furthermore, you need to treat the ㄹ batchim as if it were a no batchim. The past tense of this form is -았/었을 테니까 and, as you can see, it follows the ‘last vowel’ rule. Regardless, in both cases, you will have to keep an eye on the Korean irregular verbs since they can do some silly things. Keep an eye out on your ㄷ irregular verbs and your ㅅ irregular verbs.
Lastly, you can also use this form at the end of a sentence. It will have the form -(으)ㄹ 테니까요. The meaning remains unchanged.