I'll do it! 私がやります！

Both "will" and "be going to" talk about the future, but they are used at different moments. Use "be going to" (chapter 30) for a plan you have already decided before speaking ("I'm going to visit my family next week", the ticket is booked) and for a prediction based on what you can see now ("Look at those clouds — it's going to rain"). Use "will" (I'll, you'll, won't) for a decision you make AT THE MOMENT of speaking ("The phone's ringing — I'll get it"), for an offer ("That bag looks heavy — I'll carry it"), and for a promise ("I'll pay you back tomorrow, I promise"). "Will" is also common for opinions and predictions with "I think / I'm sure": "I think it will be sunny", "She won't be late". The key contrast: a plan already made = going to; a decision made right now = will. Learners often use "be going to" for a spontaneous choice: "Someone's at the door — I'm going to open it" ✗ → "I'll open it" ✓.

「will」と「be going to」はどちらも未来を話すが、使う瞬間が違う。話す前に既に決めた計画には「be going to」（第30課）:「I'm going to visit my family next week」（切符を予約済み）、そして今見えることに基づく予測にも:「Look at those clouds — it's going to rain」。話す«瞬間»に下す決定には「will」（I'll, you'll, won't）:「The phone's ringing — I'll get it」、申し出:「That bag looks heavy — I'll carry it」、約束:「I'll pay you back tomorrow, I promise」。「I think / I'm sure」とともに意見・予測にもよく使う:「I think it will be sunny」「She won't be late」。要点の対比：既に決めた計画 = going to；今下す決定 = will。学習者はとっさの選択を「be going to」でよく言う:「Someone's at the door — I'm going to open it」✗ →「I'll open it」✓。