What is the difference between will - be going to and Present Continuous?

Question

I used to think that Simple Future(will) describes an action that will happen in the future but we don't know when they'll happen. On the other hand, we tend to use be going to and Present Continuous when we wish to refer to a fixed arrangement; sth that is definitely going to happen at a specific time in the future because it is planned. However, today I got a little confused because of a sentence I saw in my coursebook: "I will visit my grandparents next month". Given the specific time in the future-next month-we know that we're talking about an arrangement. Then, why hasn't the book used Be Going To or Present Continuous?

Answer 1

Here's a brief summary of the subtle differences in the use of the tenses.

Present simple: for actions that are in progress at the time of speaking - eg: I am cooking dinner now. Call me later.

or which are planned for the future, usually accompanied by a time clause - eg: I am seeing him on Friday.... or: I am going to France next week

"going to" is a stand-alone tense which expresses future actions in a less formal, more loosely planned or more spontaneous way, with the emphasis on the action rather than the time.

eg: I am going to make a cup of tea. Do you want one?
or He's just lost his license so he is going to sell his car.

The future simple (will) is a much more functional tense which we normally use for specific roles like -

promises: I will buy you an ice cream if you help me.

threats: I will kill you if you do that again!

warnings: Careful. He will bite you if you get too close.

predictions:

spontaneous reactions: Me: Gosh it's cold
You: I will close the window

Jack: Who is going to feed the dog?
Jill: I will do it.

Remember that there is a certain degree of "interchangeability" between them, so these are not hard and fast rules.

In answer to your last question, perhaps the speaker was making a promise or reacting sponaneously, making changes to old plans.

eg: "You can't come to London at the weekend - you're visiting your grandparents"
"Oh yeah.....oh well, I will visit my grandparents next month"

English is a pain-in-the-a**e language, isn't it...?

Answer 2

The present continuous is used when the action is still in progress, not finished.
eg.
I am doing something.
Use the continuous form for something that is happening at or around the time of speaking.

You can use the present continuous to say what you have already arranged to do, as it is more natural for arrangements. The 'will' form should not be used for arrangements.
When we use 'going to' in a sentence it makes us believe that the action is really going to happen. It refers to the future, but it something that will happen for sure.

The future, with 'will' is used for almost everything, as long as it is in the future.
In the specific situation that you clearly stated, using the simple future or the present continuous is correct.

The best thing is to buy a grammar, as this are very specific question.

I hope I helped.