NOT A TEACHER
1. Here are two helpful definitions from Mr. Raymond Murphy in Grammar in Use:
ON TIME = not late.
IN TIME = soon enough to do something.
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I have made up these examples. They are not very good, but maybe they will help give you the idea of the difference.
The airport told me to arrive by 3 p.m. ( by = before 3 p.m. or at 3 p.m.) I arrived at the airport at 3:05. They told me:
"Sorry! You did not arrive at the airport on time (that is, you arrived late). At 3:01 p.m. we sold the last ticket to someone else. Too bad. Next time, be on time."
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My airplane was scheduled to leave at 4 p.m. Because of heavy traffic, I arrived at the airport at 4:30. But my airplane
was still there (it had had some minor problems, so it was still at the airport). I was so happy that I had arrived in time
to get my plane ( = arrived soon enough to get my plane. The plane left at 4:35! I made it by five minutes!).