I did a little searching and more than once I've found that "will" is used as opposed to "going to" if there were no previous plan and the speaker just decided it. But if that's the case, how do you explain "I'm going to go to the bathroom." That's certainly not planned but you wouldn't suddenly say to someone "I'll go to the bathroom."
Another example. Say you and your friend have been reading in a room on a hot summer day. Being the hot day that it is you wonder if it's cooler outside so you say to your friend "I'm going to read outside." You wouldn't say "I'll read outside." right?
So what's going on here?
"I'm going to go to the bathroom" indicates a future plan, but it would be a strange thing to say as you don't normally warn people a long way in advance of something like that.
"I'm going to the bathroom" could be the present continuous used as the future (ie you are sitting at a table in a restaurant and, before you stand up to head for the bathroom, you might say "I'll be back in a minute, I'm going to the bathroom".
If you bump into someone on your way to the bathroom and they ask "Where are you going?" then you would still say "I'm going to the bathroom" - this would be the present continuous in its normal form because you would at that point be in the process of walking to the bathroom.
Hmm. Interesting. I didn't consider that those might not be future tense. Here's another one then. Same scenario but you say to your friend "I'm going to get something cold to drink. Do you want anything?" This isn't present continuous right? You haven't started to get the drinks yet. But in this case as well you wouldn't say "I'll get something cold to drink. Do you want anything?"
Good morning, Saih.
(1) I, too, have wondered about what you have asked.
(2) I found something in the Longman English Grammar (by the most
esteemed Mr. L. G. Alexander) that may (I emphasize "may") gives us
an answer. (Or something CLOSE to an answer)
(a) In discussing be going- future, Mr. Alexander says:
Intention can be emphasized with adverbs like "now" or "just":
I'm now going to show you how to make spaghetti sauce.
I'm just going to change. I'll be back in five minutes.
(3) Maybe (maybe!) your sentences need a "now" or "just" to more
clearly bring out the idea of INTENTION:
Would you please excuse me. I'm just going to go to the bathroom for a minute.
I'm just going to read outside for a while. Call me if you need me.
Have a nice day!
Given that English is different in different parts of the world, it's often useful for learners to know where you live, especially given that you're an English teacher. Your answers might reflect local usage, etc.
In learners profiles, it's occasionally very useful to know in what country they are trying to learn English.
This isn't necessarily the official reason - which I don't know - but it's certainly helpful information to have in some cases.
A better question would be, 'Why do people write that "will" is used as opposed to "going to" if there were no previous plan and the speaker just decided it.'
The answer no doubt has something to do with the teaching of English to non-natives.Like most rules for beginners, they should be understood in terms of the point they are trying to communicate.
I thought of another example. Say a boy is at home when the phone rings. It's his friend and he wants the boy to help him paint his car. The boy gets off the phone and tells his mother "I'm going to help Rick paint his car." There's no question that you wouldn't use "will" for this unplanned turn of events.