The differences between these can sometimes be hard for EFL/ESL students to discern.
One way to help students choose between structures is to have them consider their level of certainty. Try dictating the examples above randomly (using appropriate inflection), and have the students put them in order based on certainty. In order to reinforce this concept, have them read each line back using proper stress.
However certainty is not always clear. For example,
“I’m going to pick Carmen up at the airport,” and “I’ll pick up Carmen at the airport.”
Generally if something is planned, we use the first example. On the other hand, when making a spontaneous decision we will use the second example with “will.”
A fun way to practice this is to have students make predictions about the future. Is the world going to end in 2012, or do they think it will end?
Simulate a meeting where the group decides what tasks need to be completed before a big event. Then students volunteer for each task. “I’ll do this. I’ll do that.” Finally they report to the teacher. “He is going to do this. She is going to do that.”
Tell your students to bring in their schedule books (or diaries). Have them tell you or a partner what they are doing, going to do, or might do.
Finally, do not forget the future progressive. This structure connotes some formality. Brainstorm announcements heard in an airport or train station. “Your flight will be boarding in 15 minutes.” “The train to Kalamazoo will be departing from track 12.”