Teaching the future in English is relatively simple in the beginning. Students understand the future with 'will' and learn the form quickly. However, the problems begin when discussing the future with 'going to'. The key issue is that the future with 'going to' is logically a better fit when speaking about the future. The future with 'going to' tells us about our plans, whereas the future with 'will' is mainly used to discuss reactions that occur at the moment of speaking and speculation about the future. Of course, there are other uses, but this main issue leads to a lot of confusion amongst students. Choosing when to introduce the future with 'will' and 'going to' carefully can make all the difference in comprehension. I choose to delay introducing these forms until students are comfortable with some basic tenses. Here is some help on how to teach present simple and how to teach present continuous, as well as the how to teach past simple (and, possibly, how to teach continuous) forms. This ensures that students are comfortable with the idea of a variety of auxiliary verbs and will be able to switch between these two future tenses with more ease.
Introducing the Future
Start by Speaking about Plans and Hopes
To help students become familiar with both forms, discuss your future plans as well as your thoughts about the future. This will ensure that you use both the future with 'will' and 'going to'. If you are teaching beginning level students, separating the two forms will help students understand the difference. If your students are intermediate level, mixing the forms can assist in teaching the fluidity between the forms in everyday usage.
I have some predictions for next year. I think that you will all speak better English at the end of this course! I'm sure I will have a vacation. However, I don't know where. I'll probably visit my parents in Seattle in the summer, and my wife will ...
Next year, I'm going to take up the guitar. It will probably be very difficult for me, but I love music. My wife and are going to fly to New York in September to visit some friends. While we're in New York, the weather will probably be good...
In both cases, ask students to explain the function or purpose of the different forms. Help students understand that the future with 'will' is used for making predictions, or what you think will happen. The future with 'going to', on the other hand, is used to state future intentions and plans.
Future with 'Will' for Reactions
Introduce the future with 'will' for reactions by demonstrating various scenarios that call for reactions:
John is hungry. Oh, I'll make him a sandwich
Look it's raining outside. OK, I'll take my umbrella.
Peter doesn't understand the grammar. I'll help him with the exercise.
Practicing Future Forms
Explaining Future Forms on the Board
Use a future with 'will' for promises and predictions timeline to illustrate the future used for speculating about the future. Contrast this timeline with future with 'going to' for intentions and plans timeline to illustrate the difference between the two forms. Write positive sentences of both forms on the board and ask students to change the sentences into both questions and negative forms. Point out that 'will not' becomes 'won't' in most every day use.
Comprehension activities focusing on specific functions will help cement the understanding in differences between these two forms. For examples, a reading comprehension on the weather can help students use the future with 'will. This can be contrasted with a listening comprehension discussing future plans with 'going to'. More extended dialogues and reading comprehensions can be used to mix the forms once students understand the differences between the forms. Quizzes asking to choose between future with 'will' or 'going to' also help to solidify understanding.
Challenges with the Future
As discussed above, the main challenge is in distinguishing between what is planned (going to) and what is a reaction or speculative (will). Add to that the fact that many native speakers mix the forms themselves, and you have a recipe for trouble. I find it helpful to boil teaching down to two questions:
Was a decision made about this statement BEFORE the moment of speaking? -> If yes, use 'going to'
Are you thinking about future possibilities? -> If yes, use 'will'
Is this a reaction to what someone has said or done? -> If yes, use 'will'
Not all uses of these two forms can be answered with these simple questions. However, raising students' consciousness of this key points will help them become more accurate in their use of these two future forms.