Difference Between “Will” and “Going To”

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What's the difference? 'Will' and 'be going to'

will ; 1. A decision at the moment of speaking: 즉석 결정

        2. A prediction based on opinion: 견해상의 예측

        3. A future fact: 미래 사실

        4. For promises / requests / refusals / offers: 약속, 요구, 거절, 제시 등

going to ; 1. A decision before the moment of speaking: 말하기 이전에 결정됨

                2. A prediction based on something we can see (or hear) now: 시각/청각적에 의거한 예측

출처      https://www.perfect-english-grammar.com/will-or-be-going-to.html

    (The phone rings)
    Julie: I'll get it!
    ('I'm going to get it' is very strange, because it makes us think that Julie knew the phone was going to ring before it did).

    I'm going to go on holiday next week.
    ('I'll go on holiday next week' makes it sound like you've only just decided at that minute. Of course, this is possible, but normally we plan our holidays more in advance!). 

Other points about the future:

We use the present continuous tense for definite future arrangements. Often, it doesn't really matter if we choose 'be going to' or the present continuous. In the following example, there is really very little difference in meaning:

    I'm going to the cinema tonight.
    I'm going to go to the cinema tonight.

We use the present simple tense in two cases. First, we use it for a timetabled event in the future, like public transport or the start of a class:

    My train leaves at six tonight.
    His class starts at 9am tomorrow.

Second, we use it after certain words, when the sentence has a future meaning. These words are: before / after / as soon as / until / when:

    I'll call you when I get home.
    She's going to study after she finishes dinner.
    Please drink some water as soon as you complete the race.

    Click here to download this explanation as a pdf.

    Try an exercise about 'will' and 'be going to' here.

출처
https://languagetool.org/insights/post/will-vs-going-to/

“Will” and “going to” are used to refer to the future. In formal writing, there’s one you should use over the other.

We’ll tell you which one.

Simple Future Tense—Difference Between “Will” and “Going To”
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In formal writing, “will” and “going to” are used differently.

Correct Use

In casual speech and writing, will and going to are often interchanged.

However, it’s important to know that in formal writing, there is a distinction.

Will is used when referring to the future with certainty and for recent, rapid decisions.

Going to is used to refer to events that have been previously planned.

“Will” vs. “Going To”

In English, there are several forms that can be used to refer to the future. The two most common are will or going to. Often, especially in casual speech, these two words are used interchangeably with no issues. However, in formal writing, they have slight distinctions. If you want to be as accurate as possible, you should know that will is more commonly used with recent decisions, certain futures, and predictions. Going to tends to be used about events that were previously decided on.

If you’re still not sure about when to use will or going to, don’t worry. We promise we will elaborate on our explanations, and that you’re going to understand them as soon as you’re done reading this blog.

Simple Future Tense With “Will”

Simple future tense helps indicate that something will happen in the future. A future tense with will is used to state something with absolute certainty.

The structure for a future tense with will is: subject + will + base of a verb.

I will eat at Fuddruckers.

The structure of a negative future tense is: subject + will + not + base of a verb.

I will not join you for lunch today.

Remember, that won’t is a contraction of will not.

I won’t be going to tomorrow’s office party.

Will is often used while making a rapid decision.

Sure, I’ll (I will) get you a slice of pizza.

Simple Future Tense With “Going To”

When using a future tense, going to is used to reference an event that has already been planned.

Last week, they decided they are going to get married in December.

Going to is also used when there is evidence in the present that emphasizes something will happen.

You’re going to fall if you don’t tie your shoelaces.

The structure of using going to in future tense is: subject + form of “to be” (am/is/are) + going to + base of a verb.

I am going to travel to Ireland for vacation next month.

He is going to feel sad if you don’t invite him to the party.

They are going to go to his parents’ house.

Please remember that in casual speech and writing, will and going to can be interchanged without changing the meaning of the sentence or causing any confusion. However, in formal writing it’s best to know the distinctions.

Correct

This year, I will finish writing my book.

Avoid

This year, I am going to finish writing my book.

Correct

I think the Los Angeles Lakers will not win the NBA championship this season.

Avoid

I think the Los Angeles Lakers are not going to win the NBA championship this season.

The Difference Between “Will” and “Going To”

Imagine if one of Queen’s hit songs, “We will rock you,” disregarded the difference between “will” and “going to.” We are going to, we are going to rock you” just wouldn’t have sounded as catchy, and the song probably wouldn’t have been quite as popular. Keep this in mind when you’re writing something formal, and deciding on whether to use “will” or “going to.”