+1
do you use will or going to for the following: Is it possible to use both.

They will get married in June.
They are going to get married in June

What are you going to do next Monday.
What will you do next Monday?
I am going to the library.
I will go to the library.
+1
It's possible to use either. It just depends on what meaning you want to convey.

An impressionistic view.

going to

hot, emotional, involved,
personal, subjective,
concrete, action, physical,
IMMEDIACY, physical evidence,
INEVITABILITY, non-contingency,
independent, "no matter what",
PLANS, INTENTIONS, DECISIONS,
warnings, threats

will

cool, cerebral, aloof,
impersonal, objective,
abstract, thought, mental,
EVENTUALITY, logical evidence,
CONDITIONALITY , contingency
("if", "unless", "when", "before", "after"),
dependent, "depending on",
SPONTANEITY,
assurances, promises, offers, refusals,
volunteering

Weddings typically involve planning, so the more likely sentence is "They are going to get married in June".
What you do in the future, insofar as you have control over it, also involves planning, so the more likely sentence is "What are you going to do next Monday?"

In the "library" scenario, it's not so easy to choose a typical case. In fact you haven't used "going to" of the future, but "going to" of motion, so your two choices are, in some sense, incomparable. "I am going to go to the library" vs. "I will go to the library" is probably the contrast you're asking about.

--What are you going to do after class?
--I'm going to go to the library. = I plan to go to the library after class.

--What are you going to do after class?
--I'm going to go to the snack shop and study. = I plan to go ...
--What if the snack shop is closed?
--Then I'll (I will) go to the library. (In the eventuality of a closed snack shop, I'll ...; On condition that the snack shop is closed, I'll ...; I haven't thought of what I would do in case the snack shop is closed, so I make a spontaneous response: I'll go ...)

Hope that helped.
CJ
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thank U
Thank you CalifJim:
It helped me a lot.
I have a question; Is it correct if I say; It will help me a lot, or, It is going to help me a lot. I think both are correct.
Both are correct.
It will help me a lot is simple, factual, and logical, and very appropriate here.
It is going to help me a lot gives more information in the context of this thread. It gives a greater impression that you plan to make use of this new information actively, by practicing it, perhaps. It suggests you already have a goal in mind, a purpose to which you are going to apply what you have learned.
CJ
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Thank you so much, Califjim!

The way you explained is really understandable.

Thank you so much again. ^^