If we compare these two sentences:
- I just done
- I just finished
The first one doesn't make sense I think while the second one does. However, if we were to remove just, then:
- I done
- I finished
Both would seem to make sense? I think. Why is this the case?
Let's take an example sentence:
I just done dinner
I just finished dinner
Done and finished are both verbs but why can't done be used in the same way as finished?
Lets note the tenses for to do, to finish and to eat (I added the last one because it's quite common and it's an irregular verb). So I'll list the infinitive, the past tense and the past participle:
To do, did, done
To finish, finished, finished
To eat, ate, eaten
Now, let's look at your first example:
I just done and I just finished
The first is obviously wrong, because done is a past participle which needs some other verb. This is not the case for finished, because it can be read as the past tense (rather than a perfect tense). Compare the following:
I just ate and I just eaten
Clearly, the first is grammatically correct, whereas the second is wrong.
The same argument can be made for your second example:
I ate and I eaten
The first is grammatically sound, the second is not. With to do and to finish this can be explained by noting that finished can be read as past tense (which is correct in this case), whereas done can only be read a past participle and the example lacks another verb to make it grammatical.
In your third example, I think both can be grammatical, but done is not used in the way you do. To finish dinner is idiomatic (people generally say that to indicate they have concluded their evening meal). To do dinner, however, is not idiomatic (it's not used by native speakers).
The base verbs are "do" and "finish", which don't mean the same thing, at least not in daily usage. The former relates to action and the latter relates to completion. I do, I did, I have done, I am doing; I finish, I finished, I have finished, I am finishing. It might be that having done something implies having finished it: "I did my homework", referring to a homework assignment as a bounded, well-defined entity, may be intended to imply "I finished my homework", but "I did some homework" doesn't imply that one finished it.
The past participles of these verbs, "done" and "finished" have both come to be used as adjectives following forms of "to be". In this sense, and only in this sense, they mean the same thing. "I'm tired. I'm done for today. I'm finished for today." "Are you still working on your report?" "No, it's finished. No, it's done."
Apparently some people think that "I'm done" isn't good writing. See https://www.merriam-webster.com/words-at-play/whats-the-difference-between-done-and-finished.
