if your friend asked to borrow $100 from you, ... you would probably not say-- you might actually put interest on this person, so you could say, alright, I'll do it with 5% interest, pay me back in a year.

你的朋友向你借100美元，你可能不会说-,你可能会对他收取利息，因此你没准会说，好吧，我要收5%的利息，一年之后连本带利还我。

耶鲁公开课 - 金融市场课程节选

Alright. I have to do this!" And then you'll say,

好。我必须做了！然后你会说，

主修古典文学 - SpeakingMax英语口语达人

which means that when you say "I'll", you are expressing your intention to do something.

当你使用 "I'll" 的说法时，你是想表示要做某事。

I will 课堂 - SpeakingMax英语口语达人