I often come across sentences like these

He kept on chasing the thief even it's deep into the night.
Shall we keep practicing in the next few days?

But as far as I'm concerned, both of the following sounds fairly OK.

He kept chasing the thief ...
Shall we keep on practicing ...?

I can barely remember that they have a monir difference in meaning but I Googled (and searched ELL.SE) around and found only usage differences. Can anyone tell the exact difference in meaning?


When you say keep doing X, it means twothings: A) you expect X to be finished soon or at a definite time, and B) you were already doing X

Keep looking at this report.

When you say keep on doing X, it means two things: A) you're talking about making X part of a routine or background activity, and B) you were already doing it.

Keep on looking at these reports everyday.

It can also mean to progress further with a path or course. In these cases you can omit the -ing word if it will be understood from context.

Keep on walking down the path until you get to the house. ("Keep on down the path until you get to the house" also works.)

Keep on driving until you are past 5th street. ("Keep on until you are past 5th street" also works.)


To the idea of "persistence" that comes from "keep" + {verb}ing, the word on adds a vague sense of forward motion (onward in space|time) and so it serves to make the abstract idea of "persistence" more concrete, which makes the statement more emphatic.

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