Hi,
early before the due, usual, thought-of or expected time.
soon within a short period of time from now or from some some previously mentioned past time time.
Now let's look at your last examples.
What if Mary arrives at 5pm instead of at 6pm?
Should she say "I'm soon" or "I'm early"? I'm early sounds better but, "soon" also means "pronto"
She's early, because she has arrived before the expected time.
You can't say 'I'm soon' in any situation, because 'soon' is an adverb.
Another thing, I can say "It's early in Canada, right?" As long as I know 'earlier than when'. eg Earlier than the time that Canadians go to bed?
- What if I said "It's soon in Canada, right?" Would it be wrong? If you mean the same as the above example about Canadian bed-time, yes, it's wrong.
It depends on your meaning and on the context. eg On December 23rd, you migtht say 'Christmas is soon'. However, it'd be better to say 'It will soon be Christmas'.
Best wishes, Clive