Unlike the languages from which it has been derived, Modern English has only six tenses. However, there are three forms of these six tenses: the basic form, the emphatic form, and the progressive form.
This progressive form is made by using the second principal part of the verb, known as the present participle [the present form with -ing added to it ] along with the auxiliary, or helping, verb to be. In order to indicate tense, or time, in this progressive form, this auxiliary verb to be is conjugated in the appropriate tense for the circumstance of the action to be communicated. The progressive form is used only when an action is continuous or repeated; it usually occurs while another action or actions take place. And, the tense of this continued action is determined by the time at which it takes (took) place.
Thus, if an action is taking place in the present, or is repeated frequently, the present progressive form is used.
Here is an example of an action that is taking place in the present:
The students are reading the short story now. (Notice that the present tense of to be is used here as the auxiliary verb)
Here is an example of an action that is repeated:
Uncle Jack is constantly telling that old joke.
If, however, there is an action that began occurring in the past and still continues into the present, the present perfect progressive is used.
Here is an example:
My friends and I have been studying for hours this evening. (This action started hours ago, but it is still occurring. And, notice that the present perfect form of to be is used here as the auxiliary verb.)