I think the idea of having an effect in the present time is a bit of a red herring (i.e. it's not what you should be focusing on). I think for sentences relating to the past the most general principle is that you start by working out what time period is expressed or implied in your sentence. If that period of time continues up to the present (e.g. "this week", "this year" etc.), then you normally use the present perfect tense, at least in formal British English (this seems to be less important in American English and maybe in some variations of informal British English - my perspective is based on British English). If the period of time expressed or implied in your sentence is wholly in the past (i.e. does not continue up to the present time), even if it ended only a fraction of a second ago, you normally use the simple past. (Let's leave the pluperfect for another time, no pun intended.) If you are describing something because of its current, ongoing effects (its effect in the present time), then often the period of time implied in your sentence would be something like "recently", which is a period of time that continues up to the present. If so, obviously you use the perfect tense. But that implication could be overridden by an express mention of past time in your sentence. The following would be grammatically acceptable in formal British English: "Last year, the claimant was involved in a car accident. She injured her leg, with the result that her employment prospects continue to be affected by her impaired mobility." The reason is that the time-period implicit in the phrase "she injured her leg" is "at the time of the car accident". In "my friend will be late for dinner because she has hurt her leg", the time-period implied in "she has hurt her leg" is (probably) something like "recently". In "my friend is punctual because she has learnt that lateness is frowned on", the implied time-period could be "in her lifetime so far", a period that continues to now.

Great questions! You were right to provide so much detail, as that makes it easy to give you the exact information you need! Correct: She hurt her leg a week ago, and she still can't walk. Explanation: We mention the time in the past, so we absolutely cannot use the present perfect. If we want, we can use to sentences: She has hurt her leg. That was a week ago, and she still can't walk. That being said, in spontaneous speech we may begin the sentence one way, and then realize we want to say it another way, resulting in a mistake as a result of changing in mid-sentence. Of course, this kind of mistake can happen with any grammar, not just verb aspect. Correct: In your memo you asked me to write a report, so now I'm sending it to you. Explanation: “In your memo” sets a time in the past — it means “at the time you wrote your memo”.

And also Present Perfect / Past Simple: if you begin your question with “When ... ?”, you have to use Past Simple. If you use adverb “just”, you use Present Perfect. But if you say “just now”, you use Past Simple. :-D I don’t understand why there is such difference between “just” and “just now”. I have only remembered this from a textbook :D Good luck with the difference between Present Perfect Durative and Present Perfect Continuos. ;-)

“A week ago” is a marker-word for the Past Simple Tense. You can use Present Perfect if the period of time didn’t end: this month, this year, this week, this morning (if it is still morning). “She has broken her leg this month”. P.S. I’m not a native speaker, but I understand your question because I had the same ones :D

Great answers from Phil. I'm just here to answer your question about whether the present perfect can be used with expressions like "last year". The answer to this one is yes, but with a qualification. You can say, "I've seen her (or "I haven't seen her") in the last year. This sets a time range. We don't say, "I've seen her last year," because in this phrasing, "last year" is a specific time, even if it's a rather vague one. In North America at least, it would be pretty common to hear people skipping the past perfect entirely and make sentences like, "She hurt her leg, so she can't walk." In fact, I believe that would be more common here than saying, "She's hurt her leg..."