For as long as I can remember, I distinguish past tense from perfect tense from the intention of the sentence. If the sentence intends to emphasise on the time, then it's past tense. If the sentence intends to emphasise on the "finish-ment" of something, then perfect tense it is. And past perfect is just the finish-ment of something at time point of reference in the past.
And then I use the rule of thumb that everything with "just" usually is having "perfect" tense somewhere in the sentence, whether it's perfect tense, past perfect tense, or future perfect tense.
My wife had just posted a picture in her instagram saying that:
"Having this phone for a year, yet I just knew my phone camera has automatic flash feature."
I corrected her that she should use "yet I have just known" instead of "just knew" because I think it's perfect tense. But she insisted that this is a past tense because the event of "knowing" is happened in the past.
Which one is true? And can you help give some explanation? Thanks.