Will vs going to with a personal skill [duplicate]
In most contexts, I am going to [verb] and I will [verb] are interchangeable. Sometimes the former may place more emphasis on the fact of your current intention/expectation, where the latter emphasises the future action.
There are some contexts where the difference is clear, and this may have some bearing on why OP thinks there's a planned/spontaneous distinction (usually there isn't). Say you're round a friend's house watching a football match on TV, and at half-time the beer runs out...
You: "I'll nip down the shop and get some more beer."
Friend: "It's raining - I'll find my umbrella for you."
If you reply "Don't bother - I'm going to use my car", the implication is you had already decided you were going to use the car before you first said you'd get the beer. But if you say "Don't bother - I will use my car", this implies you just made that decision in response to what your friend said.
@Peter Shor mentions another context ("It's going to/It will bite!") where native speakers often make a distinction. In that case, going to usually signifies immediate danger (it's just about to bite), but will can just mean it's bitten others before, and will/may bite you soon if you're not careful (effectively, the same as "Careful! It bites!" where present tense indicates "habitual" action).
Kosmonaut's "Let's say that tomorrow you will walk your dog" isn't really relevant to the current issue. It could just as well have been "...tomorrow you are going to walk..." or even just "...tomorrow you walk...". They all mean the same, and in that context there's no real reason to prefer one over another.
Doing some research, I discovered that the spontaneous vs planned rule for "going to" and "will" is taught to ESL students, but not used otherwise. I did discover a few good rules of thumb that are decent guidelines and make sense to me (a native English speaker):
"Going to" is a kind of present tense, so use "going to" in situations where the present is connected to the future. Example: I feel a drop - I bet it's going to rain. I am going to walk to school because my bike has a flat tire.
Use "will" in writing and "going to" when speaking.
- Use "will" when talking about the not-immediate future and "going to" for the immediate future.
- Use "will" and "going to" interchangeably for making predictions.
If you are looking for a good explanation about when to use either form, I suggest you read this article, which explains how to teach the differences between "going to" and "will".
Any answer given on EL&U, given the constraints of time and space, will leave out some of the truth. That perhaps explains the limited nature of the earlier answers to this question to which the OP refers. In any case, non-native speakers usually want simple answers and these will suffice up to intermediate level.
This particular example introduces additional complications, because the speaker is not talking about the speaker’s own future acts, but about someone else’s. That makes the spontaneous/planned distinction inappropriate. In this case, going to seems the most natural expression, and it is indeed ‘often used as a general verb form for the future, especially in spoken English’ (‘An A-Z of English Grammar and Usage’).
English has no set of verbal inflections to express the future. It uses instead auxiliary verbs or, in some cases, the present tense or the present progressive construction. Although going to can be used in many cases, it can’t be used in all, and it certainly isn't invariably interchangeable with will. Native speakers will know intuitively when to say, for example, ‘Next year we’re going to have a holiday in Greece’ (not will) and when to say ‘Right, I’ll see you outside in half an hour’ (not going to). There is a clear difference in usage, reflecting the speaker’s intention in each case. Non-native speakers can only expect to distinguish such differences after considerable exposure to the language. The kind of quick fix found in the spontaneous/planned distinction will serve up to a point, but not beyond.