I'm from Brasil and here we study the differences of using "Will" and "Going to" to talk about the future. But it is usually very confusing because we have a different kind of conjugation that uses no auxiliary verb, and the English Grammar has many explanations about the context we must use one or another.
So my question is. What are the real differences and needs of the colloquial usage of "Will" and "Going to"?
If the subject of your sentence is "I", then Barrie's brief explanation correctly summarizes the essential difference between going to and will. Example:
means that you have already planned what to do later.
indicates a plan made at the time of speaking.
Of course, you are not always the subject when talking about the future, and the focus may be on predictions rather than plans. In such cases there is no simple rule to follow. For example:
and
Talking about the future is one of the most difficult aspects of English grammar for non-native speakers, and includes many more than the two forms discussed here. I recommend investing in a good reference such as Swan's Practical English Usage, and asking specific questions here on this site if you are not sure which form you need in any given context.
Very briefly, you use going to about future events that are already planned, and will about future events decided at the time of speaking.
See the following sentences:
I will buy a new car sooner or later (event generically planned).
I am going to buy a new car on Monday (event precisely planned).
I am going to buy a new car (event planned without specificing the time in the future).
Also, in colloquial/confidential speaking you can use present continuos:
We are meeting outside the station tomorrow (note 'tomorrow').
She is going to the cinema with John tonight (note 'tonight').
