I would be extremely grateful if you could clear out my doubts about which tenses to use with the conjunction of time -or reason- "since".
1) I'd like to know which tense we can use (in academic English and not in everyday speech) with the time conjunction "since". Le simple past is unanimously chosen but what about the present perfect?
a) It is / has been a long time since I SAW her. 100% correct
b) It is / has been a long time since I HAVE SEEN her. (also academic ?)
(by the way are both forms "It is a long time" and "It has been a long time" formally accepted?)
c) It's been a week since I SAW my friends / I GRADUATED.
d) It's been a week since I HAVE SEEN my friends / I HAVE GRADUATED.
e) I've been cheating on you since I KNEW you. (?)
f) I've been cheating on you since I HAVE KNOWN you.
g) How long is it since you KNEW her ? (??)
h) How long is it since you HAVE KNOWN her?
i) How long HAVE you KNOWN her ?
2) Is it true that we cannot use the present tense in the main clause when "since" expresses time but can only use it when "since" introduces a reason? However, let's look at the following sentences, can't they be made of time clauses?
a) She LOOKS quite different since her illness. (time clause ?)
b) He IS SHOOTING the most expensive film since Star Wars. (correct ?)
c) My mother LOOKS younger since she dyed her hair. (time or reason clause ?)
d) He LOOKS much younger since he shaved off his beard. (time or reason)
e) She no longer VISITS since she got married. (time or reason?)
At some point I thought that the present tense could be used with the time expression "since" only with state verbs but "visits" above is a verb of process, so what is the correct answer?
Thank you very much for your help.