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According to my grammar, when we talk about predictions, "will" can be used when we are certain something is going to happen and "going to" for evidences. I think that when we are talking about evidences, we are certain something is going to happen. Am I not right?

When the forecasters forecast the weather, I believe they use "will". Well, they are almost 100 sure and they say this because they have some evidence about this because of the satellites and stuff. So why not "going to"?

When I am 100 % sure about predictions, I use "going to" and when about true facts I use will.

Look at the sky. It looks as if it's going to rain.

His birthday will fall on August 12th.

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marked as duplicate by Edwin Ashworth, Bradd Szonye, user49727, MrHen, Kristina Lopez Oct 3 '13 at 21:53

This question was marked as an exact duplicate of an existing question.

I'm not sure what you mean by "evidence" here, but "going to" is a statement of intent, not a kind of evidence. And "will" does not always indicate certainty. – Bradd Szonye Oct 3 '13 at 9:57
This has been addressed many times - I think StoneyB's comment at english.stackexchange.com/questions/87900/going-to-vs-will should be read first. Then Peter - Neil - Peter. – Edwin Ashworth Oct 3 '13 at 10:00
Your grammar writer has done something which grammar writers often do: they try to find a "rule" which they can put in their grammar, and end up saying something which is just not true, or at least only approximately true. – Colin Fine Oct 3 '13 at 17:28
@Colin Fine: It should be a rule that your statement (from '[S]omething' ) has to be included at the front of all grammars. – Edwin Ashworth Oct 3 '13 at 22:14

Be going to is used to talk about plans and intentions, but it is also used, as you say, to refer to some future event when we have some evidence that it will happen.

When we’re referring to ourselves or others, will often describes future intentions decided on at the time of speaking. Be going to, by contrast, often refers to an intention we already have.

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