The Simple Tenses are used with serial situations:
The Simple Tenses are also used with permanent situations:
The Past and Future Simple can refer to singulary situations:
But in the present we are in the middle of a singulary situation, so we have to use the Progressive:
The Present Simple is used to talk about general truths (e.g. scientific facts):
We use the Present Simple to talk about the contents of books, films etc. (because they generally don't change):Hawkings gives a list of standard forms in Chapter 2.