When we speak, we choose the tense that we use. This is important in English, because the tense we choose expresses more than just a simple fact. When we speak about the future, the tense we choose can express how we “see” the future, even our personal feelings about the future. It certainly expresses what we believe to be the probability (the chance, the reality) of something happening or whether we have already decided to do it.

This table gives a simple scale of probability for each structure. It is not exact because language is not a science, and there are many variables. This table should help you to think about the “concept” of the future in English. This concept does not exist in all languages, but it is rather important in English.

% probability before speaker speaks of event happening structure used for example
0% will no plan Don’t get up. I’ll answer the phone.
70% going to intention We’re going to watch TV tonight.
90% present continuous plan I’m taking my exam in June.
99.999% present simple schedule My plane takes off at 6.00am tomorrow.
It is impossible in English to express the future with 100% certainty. (The speakers of any language that can do this must all be billionaires!)