A quick answer to your question about the use of will
vs. the use of be going to is that will
expresses an action the speaker is willing to perform and has the intention
to perform; the speaker may have just decided to express his/her intention.
On the other hand, going to expresses something that
the speaker has already planned to do. It’s necessary to put your
sentences in context to see the difference.
One situation in which will (but not going
to) is appropriate is to make an offer:
Don’t carry that big bag of groceries, Mrs. Jones. I’ll
take it for you.
If you want to sell your car, I’ll buy it
for $10,000.
I’ll go to town—to the pharmacy—right
now if you need that medicine immediately.
Another situation where will (but not going
to) is appropriate is to make a promise:
I’ll finish this work by 7:00 tonight.
I’ll call you later.
I’ll go to town for the big celebration if
my team wins the championship.
On the other hand, you can use going to (but not will)
to express a prior plan:
We’re going to take our vacation in the winter
this year, not the summer. We’ve already planned a trip to Antarctica.
Sorry I can’t be at work next week—I’m
going to have minor surgery.
I’m going to go to town next Thursday—my
class is having a reunion at the Central Hilton. I’ve bought
a new outfit and I’m very excited about it.
The last three sentences are also frequently expressed with the present
continuous—We’re taking, I’m having,
and I’m going.
Some clarifying examples appear in a paper, “Future Shock”
by Marilyn Martin (the paper appeared in On TESOL ’78: EFL
Policies, Programs, Practices. Washington D.C.: TESOL, 1978).
Martin notes the difference between the following two conversational
exchanges:
|
(a)
|
A: What about your mother’s birthday?
B: I’m going to get her a new mop.
|
|
(b)
|
A: What about your mother’s birthday?
B: I’ll get her a new mop.
|
As Martin explains in sentence (a), “speaker B is reporting a
prior decision,” while in sentence (b), “he is making what
seems to be a spur-of-the-moment decision as he speaks.”
She continues:
“In order to test this intuitive judgment, however, let us consider
the conversation one could have with oneself, as in
What can I get my mother for her birthday? Oh, I know! I’ll
get her a new mop.
X Oh, I know! I’m going to get
her a new mop.
There emerges here the impression that the be going to future
carries a sense of prior decision that is lacking in will.”
This is only one example of a difference
between will and be going
to. There are other meaningful factors to consider when choosing
the ways to express future time (Martin clarifies them in the article
mentioned above). For the question that Batya asked, however, it seems
that the choice can be made based on whether the context expresses volition
(one's choice, intention at the moment, will) as in "I’ll
go to town" or the intention to do something because of
a prior plan as in "I’m going to go to town."
See also a related discussion
in the message on "Will."