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摘一段Ron的话
if you are using the present perfect for point actions -- i.e., for things that happened at a definite time in the past and don't persist (although their effects may persist) -- then this is a non-issue in the first place, because those events occurred at one point and that's it. therefore, in the case of such events, the present perfect must refer to the impact of the event on the present situation.
for instance, if i say i have earned a graduate degree in chemistry, then this is obviously something that happened in the past; the use of the present perfect indicates that it is somehow relevant to the current situation.
on the other hand, if you are talking about states, conditions, titles, jobs, etc. -- i.e., things that persist over a period of time -- then there are two possible uses of the present perfect:
1/ (IF A SPECIFIC TIMEFRAME IS MENTIONED) continuing into the present. e.g.,
i have been self-employed for 18 years. (this means up until now; notice that "for 18 years" is a specific timeframe)
i have been lonely since she left (up until now; "since she left" is also a specific timeframe)
2/ (if no specific timeframe is mentioned) these things have been true in the past, but aren't anymore. e.g.,
i have been self-employed. (without the timeframe, this means i'm not self-employed anymore. if i were still self-employed, then i would either use a timeframe, like #1, or write "i am self-employed")
i have been lonely. (i don't feel this way anymore. if i do, then i have to use a timeframe, as in #1, or use the present -- "i am lonely.")
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