in是指在...之內，所以不難理解in time可以是指提前完成某事，是及時；on則是指在某一時間點上，是個確切的時間點，所以on time是指準時。

in time的例句

Tom managed to get to the airport in time.

湯姆設法及時趕到了機場。

There's a slight chance that he'll be back in time.

他及時趕回來的可能性不大。

I hope we get there in time.

我希望我們及時到達那里。

You should return the book in time.

你應該及時還書。

It's good to go to school on time.

按時上學是好的。

on time的例句

She always pays her bills on time.

她總是按時支付賬單。

We worked like crazy to get it done on time.

我們拼命地干，好按時完成這項工作。

The bus came right on time.

公共汽車正好準時到達。