present perfect 1

[bayan said]

Hello!In this messageI`ll ask one big & irritating question ..In this question I`m dealing with a very important & difficult matter:present perfect,
in fact I`ve understood the whole grammar of this tense but have one idea that hasn`t been clearified by any teacher or even expert I`ve met&I hope you`ll answer fully & make me understand it but befor that please make all your talk & explanations about British English only no thing to do with AE.( I`ve read"the exact time at which an action happened isn`t important"what did the writer mean by"it isn`t important"& when is it important?, on the other
hand I`ve read"sometimes the speaker mayn`t mention a specific time but they do have one specific time in mind"how do we know that the speaker has that specific time in mind if the time isn`t mentioned?this is point1.point2 is that I`ve had several examples which I don`t know why the past simple/present perfect is used.-A-"I`m very angry because he shot my dog"
why didn`t the speaker say "because he`s shot my dog",as there`s a present result?-B-compare1-"I`ve accepted an invitation to Terry`s house for dinner"but"I received an invitation to Jannet`s party,I can`t wait to go!" why is the present perfect used in the first one but not in the second one?
-C-why is it possible to use both the past simple & present perfect in this sentence"I`m afraid I tore/have torn the news paper so it`s difficult to read"
D-"Have you ever seen a magic show?~no,but my sister saw David copperfield"why is the past simple used with"saw"& not the present perfect although the time isn`t mentioned"-E-"I`ve drawn several pictures today,I drew them on drawing-paper"when I asked an english teacher why we used the past simple in the second one but the present perfect in the first one,he said"because when talking about a verb that has been already talked about we use the past simple & because "on drawing-paper"so what did he mean by"because of 'on drawing-paper'"?-F-"we`re engaged to be married,you`re the first people we`ve told -except for my parents-Why is the present perfect used with "told"-G-I`ve read"we`re happy because you`ll marry our daughter.We give our consent to your engagement.Though we`ve only met you once when you came here for a short time last christmase "why is the present perfect used with"meet"although there`s a specific time (i.e when you came here for a short time)-H-"In different parts of England,there`re different pronunciations"~"yes!I noticed that"why didn`t the speaker say "I`ve noticed that",since he didn`t mention when & because I`ve met some phrases like"I`ve noticed etc.."?( there`re other examples in the next message)thank you very very much for answering in advance.