Discuss the differences betwen the use of PRESENT PROGRESSIVE and PRESENT PERFECT PROGRESSIVE structures in English.Illustrate answer with examples.

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.)