That was nicely put, jws. Quite nicely, indeed.4. I define the Present Perfect tense as expressing an action that is already COMPLETED at the present time, OR an action BEGUN earlier and spanning a period to the present time.
Like in, "I have lived in L.A. since 1999" wherein 'have lived' expresses a span of living in L.A. The span started in 1999, hence '-ed', and continues up to the present time, hence 'have'.
Mind you, continuity will vary with regards to verbs such as "read" and "die" for example.
Cas1. I have read that book.
Cas2. I have died and gone to heaven.
Unlike 'live', 'read' and 'died' do not continue up to the present. Those events are over. So, given our current definition, which I will adopt from you, we now have to deal with the question: How do you deal with verbs like "have lived", which are not over at the Present time (i.e. I have lived in L.A. since 1999 and I am still living in L.A.)?
Which brings me to my humble point. :D
It's not the action, per se, that's over, it's the timeframe that is completed (by...): *note 'completed' is a past participle; 'complete' is an adjective.
X is completed by Y (X = past time, Y = present time).
Past time starts the span, whereas Present time completes (i.e ends) the Past timeframe, not the span. Time ends, specifically Past time, and not necessarily the action subsumed within that timeframe. Hence the grammaticality of "I have lived in L.A. since 1999."
In other words, I'd put my money on (a), not (b):
Cas(a) "It (the Past timeframe) is completed (by the Present timeframe)"
Cas(b) "It (the action) is completed by (?)
What is (?) ? Learners come across this, too. (b) is a problem.