• The most common use of will is as an auxiliary verb to show future time. It expresses a future fact or prediction: I'll be 30 in a few days' time. It will be cold and wet tomorrow.
• Will for a prediction can be based more on opinion than fact or evidence — often with expressions such as I think, I hope, I'm sure.
• Will is common in the main clause when there is a subordinate clause with if, when, before, after, as, etc. Note that we don't use will in the subordinate clause: You'll break the glass if you aren't careful.

Going to can express a prediction based on a present fact — there is evidence now that something is sure to happen.

• Will expresses a decision or intention made at the moment of speaking: There's someone at the door. I'll phone you back in a minute.
• Going to is used to express a future plan, decision, or intention made before the moment of speaking: When she leaves school, she's going to train as an actor.

• Will as a prediction is an auxiliary verb that simply shows future time. The word itself has no real meaning.
• Will is also a modal auxiliary verb and can express a variety of meanings: offer (I'll help you carry those bags.), willingness (Will you marry me?), request (Will you open the window?), refusal (My car won't start.), promise (I'll love you for ever.), prediction about the present (The phone's ringing. I'll be for me.)
• Shall is found mainly in questions. It is used with I and we: Where shall I put your tea? What shall we do tonight? Shall I cook supper? Shall we eat out tonight?

• The Present Continuous is used to express personal arrangements and fixed plans, especially when the time and place have been decided: I'm having lunch with Mark tomorrow.
• The Present Continuous is used to refer to arrangements between people. It is not used to refer to events that people can't control.

• The Present Simple refers to a future event that is seen as unalterable because it is based on a timetable or calendar: My flight leaves at 10.00. Film starts on April 4.
• It is used in subordinate clauses introduced by conjunctions such as if, when, before, as soon as, unless, etc.: We'll have a picnic if the weather stays fine.

• The Future Continuous expresses an activity that will be in progress before and after a time in the future: Don't phone at 8.00. We'll be having supper.
• The Future Continuous is used to refer to a future event that will happen in the natural course of events, uncoloured by ideas such as intention, decision, arrangement, or willingness: Stay and meet Pat. He'll be arriving any minute now.

The Future Perfect refers to an action that will be completed before a definite time in the future.