Going-to future tense is one of the four future forms used in English. It is used more often than 'will' and indicates a plan which has been made for some time in the future, near or far. It is also used to make a prediction based on circumstances evident at the time of speaking which indicate that something is certain to happen.
The sentence structure used to express the tense is to use the correct form of the verb "to be" + going to + infinitive. An example of this in affirmative sentences is "I am going to play football this afternoon." In the negative, this becomes "I am not going to play football this afternoon" and in question form, it becomes "Am I going to play football this afternoon?" The use of this form of future tense indicates that the plan has been made but maybe there have not been any specific arrangements made as yet.
Going-to future can be replaced by the present progressive tense when using an expression of time. Using the same example, this then becomes "I am playing football this afternoon." This is especially the case when the sentence contains the verbs "go" and "come." Rather than saying “I am going to go to the dentist at 3 o'clock," it is easier and perhaps a little more elegant to say “I am going to the dentist at 3 o'clock."
In making a prediction based on evidence, the same structure is used. An example of this case is "Look at those clouds. It is going to rain." Going-to future is used when there is little doubt that something is going to happen in the very near future as in "Look out! You're going to crash!"
The other three future tenses are will-future, present progressive and simple present. Will-future can also be used for making predictions but these are usually in the longer term and not based on undeniable evidence in front of the speaker at the time of making the prediction. The prediction is about things or facts the speaker believes to be true about the future. The sentence can also be modified with a "probably" or "might" if things are not so certain. This form is also used to make assumptions, promises and spontaneous decisions.
Present progressive is also used to talk about things that have been planned for the future but with the difference that specific arrangements have already been made. Tickets have been bought, phone calls made and invitations mailed. There are times when this form and going-to future are interchangeable. Present simple is used to talk about events in the future which work according to a timetable or program. For example, "The sun rises before 6 o'clock in the summer."