Does the "Future will "and "Be going to" actually have a difference between them?

Question

Well, I might give up trying to understand the difference between those verb tenses.

I know the differences between them

Be going to: Plans planned; Predictions that you can see, and you have facts for that;

Will - Choices decided at the moment of speaking; Predictions that is not certain;

The problem is: I don't understand when to use them (Eg. : Well, I decided I'm going to/ will study tommorow) and I don't know why someone used "going to" or "will" (even if the "rules" are already specific and "easy")

And sometimes, they are interchangeable. How can I say they are are interchangeable?

And besides, there are some people who actually don't know that this difference exists

Thanks in advance. I just don't want to torture for grammar, which, at first, appears "that easy"

Answer 1

I think your understanding is correct. In most cases "I am going to" indicates something already planned, "I will" suggests a decision just made.

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Example 1:

Person A: "Hey, could you send this letter for me as soon as possible?"

Person B: "Okay, I'll post it this afternoon." - this sounds correct.

Person B: "Okay, I'm going to post it this afternoon." - this sounds odd, like you were already planning to do that.

Example 2:

Person A: "Could you give me a lift to work tomorrow?"

Person B: "Sorry, I'm going to cycle tomorrow." - sounds correct.

Person B: "Sorry, I'll cycle tomorrow." - sounds odd.

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Instances where they can be used interchangeably would be things like where there is no definite action planned or being taken by anyone, but rather something that will simply occur. Example:

"I think that football team will win tomorrow."

"I think that football team are going to win tomorrow."

Both are fine.