Use will for decisions made as you speak, predictions based on opinion, offers and promises: I'll help you. Use going to for plans made before speaking and predictions based on what you can see now: I'm going to study medicine. Look at those clouds: it's going to rain.
Same situation, two choices. Pick an example.
Maybe I'll go to Greece.
The decision happens at the moment you speak, or it's your opinion about the future.
I'm going to go to Greece. I've booked it.
The plan or the evidence already exists now and points to the future.
| Rule | Will | Going to |
|---|---|---|
| Form | I'll go · Will you go? | I'm going to go · Are you going to go? |
| Decision | Made at the moment of speaking | Made before speaking |
| Predictions | Opinion: I think, probably, I'm sure | Evidence you can see now |
| Also for | Offers, promises, requests | Intentions and plans |
It's hot in here. — I _____ the window.
Paste an email or an essay. Clarigram marks the mistakes you actually make, shows the rule behind each one as a picture, and builds your practice from them.
For arrangements with a time, a place or other people, yes: I'm meeting Ana at 6. It sounds more fixed than going to.
Gonna is how going to sounds in fast speech. It's fine in casual messages, but write going to in emails, essays and exams.