What's the difference between "early" and "soon"?
I started soon.
I started early.
I think these two are differenct, but can't explain...
"I started soon" is not grammatical; the problem is that "started" is past tense and "soon" usually refers to the future.
You can say,
"I will start soon" or
"I will start early"
The difference in these cases is that "soon" refers to closeness in time between the present moment and when you will start. "early", in this case, means early in the day.
"I am starting soon" or
"I am starting early"
The difference in the above is that, again, "soon" refers to closeness in time between the present moment and when you will start. "early", in this case, implies "earlier than usual".
You can also have something like this:
"I was offered the job and started soon after"
Again, the "soon after" refers to closeness in time between the offer and when you started. There is no way to substitute "early" in the above example.
The word "soon" means "not long (in the future) from now; in or within a short time; before long; quickly". It indicates a future time (one not far off). Thus "I started soon." makes no sense. You've got a sentence in the past tense with a future time frame. An event can't be in both the future and the past (except, perhaps, in science fiction).
The word "early" means "near the beginning of a period of time, or before the usual, expected or planned time". You'd say "I started early." if your normal start time was 8:30 but you started at 8:15 or if you began writing your essay in the beginning of the term instead of waiting until the night before it was due.
I start soon - I am going to commence doing something shortly (1 minute or 5 minutes from now) relative to the hour. It's 5 o'clock, so I will start doing something at 5.01, 5.05.
I started early - I started is past tense. Early is the opposite of late. E.g. I normally start work at 9am but this morning I started early. I started at 8am. Tomorrow morning, I start late at 10am.