Sometimes it happens that the different tense stands for the different time situations which are explained as follows.
Present continuous tense for future time situation
Present continuous tense is often used for the future time situation. You will learn it from the following examples in which its future tense alternative is also given with the same meanings.
- I am going to explain it.
I shall explain it.
- He is going to play his role.
He will play his role.
- They are going to have an experiment.
They will have an experiment.
Present tense for past time situation
It is used in newspaper captions. You have often seen it reads like that.
- The plane crashes on the hill top.
Since this is the news and the crash has already been happened. Captions are usually in present tense but they stand for the past time situation.
Many times simple present tense stands for future situation. Now look at the following sentence.
- Olympic Games start tomorrow.
It simply means Olympic Games are going to start tomorrow. It has nothing to do with present. It also shows the possibility of Olympic Games to be held.
Simple past tense for future situation
Now we can convert the above example into past like that,
- Olympic Games started tomorrow.
It looks odd but there is nothing wrong with the sentence. It gives meanings. Now the question is what is the difference between both examples, the one with present and the other with past tense. The difference is of possibility. In "Olympic Games start tomorrow." there is a possibility that games will be held whereas in "Olympic Games started tomorrow." conveys the different meaning that games were scheduled but now they are postponed. So there is remoteness of possibility in the second sentence.