With examples, describe the diferences in meaning between the use of SIMPLE PAST and the PRESENT PERFECT structure in English?plz answer in detail

In Modern English there are three forms of six tenses.  Of
these six tenses, three are considered simple tenses: the present, the future, and the
past.  They are called simple because there is only one verb used with the exception of
the future which has will before the present form.  Here is the
verb to see conjugated in the  simple past
tense:

I saw                 We saw              SIMPLE
PAST TENSE

You saw              You
saw

He/She/It saw      They
saw

On the other hand, the perfect (which is Latin for
completed) tenses are compound tenses formed by using the
appropriate tenses of the verb to have with the past participles of
the main verb.  Here, then, is the verb to see in the present
perfect tense:

I have seen           We have
seen

You have seen        You have
seen

He/She/It has seen  They have
seen

Now, the difference between the simple past tense of a
verb and the present perfect tense is that the action of the simple past
is both begun and completed in past time.  But, the action
of the present perfect tense expresses an action that has begun in the past and is
completed in the present. 

Here are
examples:

Yesterday I saw the woodpecker in
our backyard tree. (SIMPLE PAST: All action is begun and ended in the
past.)

I have seen the woodpecker come into
our backyard tree all this week. (PRESENT PERFECT:  The action began in
the past few days and is completed in the present.)