Is it correct to say for sure that we use "just" with Present Perfect and "just now" with Past Simple and it's incorrect to do it vice versa? For example:
- I've just done it.
- I did it just now.
I've just done it.
I did it just now.
Those are both correct. However...
I've just now done it.
I've done it just now.
I just did it.
Those are also correct1. What isn't correct:
I've done it just.
I did it just.
You can't use just alone in that position after the subject. That has to be just now. However:
I've done it, just.
I did it, just.
Those are fine, but have a different meaning, at least in the dialects I am most familiar with - just in that case has the meaning of barely. The other cases of just without now can also have that meaning, though it would be differentiated from the very recently sense by stress or context. Navigating dialects never being straightforward, I am informed that this use of just can also mean the same as just now in at least one regional dialect of British English.
1: Which is more natural or conventional is heavily dependent on dialect, and I don't just mean American English vs British English - regional dialects play a big part.