Present perfect goes with adverbial expressions indicating a period of
time including the present:

I have worked here since 1995. (including now) vs.
I worked there from 1995 to 1999. (not including now)

*'I worked here since 1995' is not grammatical.

I have eaten 3 pieces of pizza so far. (up to now)

Present perfect does not go with adverbials that indicate a specific
point or period of time in the past:

*I have seen her yesterday.
*I have seen him a month ago.  but

I saw him yesterday.
I saw him a month ago.

Some adverbials occur with both, in which case the perfect has the
'current relevance' sense Bruce DeSpain talked about, or the adverbial
includes the present moment:

I saw/have seen her today.
I ate/have eaten lunch already.

("ate already" sounds funny to me but I hear my native-English-speaking
students use it a lot).

This information comes from QUIGLS 'A comprehensive grammar of the
English language' section 4.23, pp.194-195. There are probably more
subtleties discussed in that book. The present perfect has numerous
subtle uses.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Johanna Rubba   Assistant Professor, Linguistics
English Department, California Polytechnic State University
One Grand Avenue  • San Luis Obispo, CA 93407
Tel. (805)-756-2184  •  Fax: (805)-756-6374 • Dept. Phone.  756-259
• E-mail: [log in to unmask] •  Home page: http://www.cla.calpoly.edu/~jrubba
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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