I'll do it!

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" ✓.

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" ✓.