(I learned this Grammer Form from GO! Billy Korean, he explained it in more detail, so if you want to watch his video you can search his name in YouTube and watch his Live Class Abridged or Live Stream Videos.)
테니까 Conjugation: Verb Stem + 을 (when verb stem ends with a consonant) or ㄹ (when it ends with a vowel)
You can also use this Form with Past Tense Verb Stem ( EX: 갔을 테니까)
It's translated to "Because" because of the - 니까 form
Used for making an assumption (I assume..., I think that/so... ")
The "thing" you are assuming will come before - 테니까 > I will do it so don't worry; Don't worry, because I'll do it: 제가 할 테니까 걱정하지 마세요.
텐데 Conjugation: Verb Stem + 을/ㄹ 텐데
Shows Contrast between two sentences
You can also use this Form with the Past Tense Verb Stem (했을 텐데...) ((I) assume he/she did it)
(You can also use 했었을 텐데; it's "double" past tense, so it means had done)
(제가 했었을 텐데: I would have done it...)
It can mean and, but, so..., though
(if I'm not mistaken this contrast comes from the - 는데 Form, this Form is seriously annoying me lol)
Like - 테니까, - 텐데 also shows an assumption > 이제는 너무 늦었을 텐데: I assume it'd be too late now; now I think it'd be too late >많이 추울 텐데 괜찮겠어요? I assume it's gonna be really cold, are you gonna be okay? (춥다 is a ㅂ irregular, so pay attention to its conjugation)
You can use 제가 할 테니까, but using 제가 with 텐데 (제가 할 텐데) will sound awkward