Be Going To - Simple Future

I've just finished teaching an intemediate class and I've noticed that some students were mixed up about Simple Future (Be Going To) and Present Continuous (Be + Verb + ing). Well, I had posted about this subject in May 2009, but it's about time to updated it:
Simple Future is used when you have made a decision, you have taken a plan, you have an intention. It's formed as follows:
Verb to be + going to + verb in its infinitive form
I'm going to correct a lot of tests this afternoon.
She's going to have dance classes on Monday night.
They're going to do their homework.

In their negative form, you have:
I'm not going to correct a lot of tests this afternoon.
She's not going to have dance classes on Monday night.
They're not going to do their homework.

And in their interrogative form:
Am I going to correct a lot of tests this afternoon?
Is She not going to have dance classes on Monday night?
Are they not going to do their homework?

I've twitted some good tips to my Inter level students and here you have the links: http://www.colegioweb.com.br/ingles/be-going-to-future and http://www.inglesonline.com.br/gramatica-basica-resumida/futuro-be-going-to . On these two links you find more explanations, but on the English-Hilfen, English4U and Web2 (just click on the words) you can do exercises on line. Well, I'm sure that I've helped you. Enjoy!