1)"I have been preparing food all day long and now I have prepared it",
11, in colloquial everyday chat: “...but now I’m done” or “...and now it’s ready"
"I had been preparing food for 6 hours yesterday (today), and now I have prepared it"?
Subtle difference: ”I had been preparing” suggests that your work had somehow been interrupted but maybe you resumed it today: “...when Mom insisted I quit, so this morning I finished up"
Does the meaning change in these two sentences if we say" ... I have prepared it by now"
Also subtle diff: “by now” suggests you had plenty of time for the job and could even have even finished earlier. You might say, “....so by now it’s ready."
or "I have prepared it just now”?
No. we’d say “I finished just now"
Can we use the pres. perf. with "now", "just now", "just", "by now” ........?
//////the pres. perf. with "today”?.......
Sm, I’m no linguist but maybe this will help
11, slight differences in construction often indicate what I’ve called “subtle” differences in meaning. Hang in there, it takes years to catch on; though many of use never reach that pinnacle of achievement