Stephen,
You're getting hold of the wrong end of the stick as loswhites pointed out. "Before " goes up to the present, and it is not a definite time reference anyway.
You couldn't even suggest an example of your own!! So, "I have seen him before" means the seeing 'goes up to the present'. Is this what you want to say? To tell the truth, if another ONE MORE person here agrees with you in this case, I also will have to. I really don't mind. Present Perfect is piece of cake, but also very complicated if you don't get the knack. I am afraid you are wrong about the use.
...and it is not a definite time reference anyway.
I hope you can be more careful with your definition -- perhaps this is why you didn't write one. TOMORROW is also a
definite time reference, but it doesn't often stay with Present Perfect. In other words, merely
definite time reference is not a necessary condition of using or not using Present Perfect.
I guess you actually wanted to say
definite PAST time reference. However, the Past Family such as "in the past xx years" can be as specific as down to five, four, or three and years, months, days, minutes, or even seconds. Is it specific or definite enough? If not, please tell me how to define your
definite PAST time reference. Also, members of the Past Family each have the adjective
PAST, so they are
PAST enough. It is as
PAST as your forgotten 'PAST' in
definite PAST time reference. It is the same
PAST. Therefore, we will have a very hard time not to admit the Past Family as
definite PAST time reference.
Look at these two examples:
He's done some work for us this year.
He did some work for us last year.
In both cases the action took place in the past, but the present perfect can be used in the first case because this year is not yet finished.
Very ingenious. We know that
this year can be coupled with ANY tense:
Ex: I study Chemistry
this year.
Ex: I am studying Chemistry
this year.
Ex: I will study Chemistry
this year.
Ex: I studied Chemistry
this year.
Ex: I have been studying Chemistry
this year.
And you use
this year solely to explain Present Perfect, so that you see a
fine match.
At last, let's suggest I have
again got hold of the wrong end of what you have in mind. Could you now shape up a careful definition for using Present Perfect? Comparing with the golden rule "Present Perfect doesn't stay with past time expressions", I really want to see how much better your definition will be. Try to break down the definition into a few points. This will help, I suppose.
Shun