Thank you very much for your explanation. And thank you for your encouragement.
He can't say "keep". Here "keep on" means "keep on working". Then he can also say "Keep working". Is that right?
Looking forward to hearing from you.
Thank you in advance.
Have a nice weekend!
***** NOT A TEACHER *****
Good morning, Jiang.
(1) Thank you for your kind comments.
(2) As you can see, your question has certainly made native speakers think more deeply about their language.
(3) Yes, "Keep on" would = "keep on working," while it would sound very strange, indeed, if one simply said, "Keep." In that case, it would be mandatory to add "working."
(4) As Teacher EMSR pointed out, in reality, most native speakers would probably not be that comfortable in saying "Keep on" without the -ing, but I personally think the workers would understand that there's a missing "working." The boss would simply wave his hand and command "Keep on."
(5) That professor gave this example of "keep on" without a following
I think after the initial check's been made it's important to KEEP ON and maintain a check on it. ("Keep," he says, is not possible.)
(6) That professor says that "keep on" = persevere, carry on (as Teacher EMSR said), not give up, continue.
(7) One very nice poster also mentioned that collocation MAY play a part. His post was very informative and thought-provoking.
(8) The professor so correctly wrote that this matter of "keep/ keep on" has been almost completely ignored by English reseachers.
(9) I think his main point is: the dictionaries seem to equate "keep" and "keep on." He believes that this is too simple of an answer.
(10) If you come across further information, please do share it with us.
One is always learning more about his/her native language.