FUTURE

WILL

Will is classified as a modal verb (like can, would, could, should) therefore it has the same characteristics:

It does not change in the third person (he, she, it).
It is always combined with another verb in the base form (without 'to').
We don't use it with 'Do' in questions or negatives.

When to use WILL

We use will to talk about the future in sentences with expressions such us when, as soon as, until, probably, before, after. Example: I’ll call you as soon as I get to London.

To talk about future facts. Example: The president will visit Spain next November.

To make predictions about the future or talk about something we think that will happen based on personal judgments or opinions. Example: I think he’ll win the election.

We use will for instant decisions that we take at the moment of speaking, in a spontaneous way. Examples: Which one? Um, I will have the chicken sandwich please. Oh, we don’t have sugar. Don’t worry, I’ll buy some.

To make promises and refusals. Examples: I will help you whenever you need me. I won’t lend him my car.

To make an offer, a promise or a threat. Examples: I will do my best to help you. I’ll take you home if you want. Don’t tell the secret or I’ll kill you.

For a habit that is a predictable behavior. Example: He will give up if he starts losing. He always does that.

In the negative, we add NOT to the end of WILL (= will not). Example: I will not stay in London for more than one month.

It is possible to use contractions in both positive and negative sentences. With positive contractions WILL becomes 'LL and is joined to the subject. Example: I’ll call you tomorrow morning.

With negative contractions, will not becomes won't. Example: I won’t stay in London for more than one month.

To form a question using WILL, we reverse the order of the subject and WILL. Example: Will you stay in London for more than one month? Yes, I will. No, I won’t. Where will you go tomorrow? I will go where the wind blows.

BE GOING TO

We use this structure with expressions such as tomorrow, next week/ month/ year, soon, tonight, in a few hours/ days, by next week etc.

To talk about plans, intentions or future events that are already planned or decided before the moment of speaking. Example: Where are you going? I’m going to play football.

We can also use the present continuous or be going to for future arrangements. In this case, we must always include when (at 7, this evening, next month, etc.) in the sentence. Example: I’m meeting him at 7. I’m going to meet him at 7.

When there are definite signs or evidences based on the present that something is going to happen. Example: Look at the sky. It’s going to rain.

When something is about to happen. Example: Get back! The bomb is going to explode.

Are you going to travel this summer? Yes, I am. No, I am not. Where are you going to travel this summer? I’m going to New York in August. I’m not going to London in August; I’m going to New York.

As you can see, both Will and Going to can be used for making future predictions without having a real difference in meaning. Example: I think it will be foggy tomorrow. I think it is going to be foggy tomorrow.

If someone asks: "Are you busy this evening?"

If I respond: "Yes, I'm going to the movies." I use going to because it is a plan I made earlier (before I was asked the question). - In this case we cannot use Will.

If I haven't made plans, then you can say either: "I will probably watch TV." OR "I'm probably going to watch TV."

Both will and going to are possible in this situation because we are predicting what will happen (since we haven't made any plans).