posted by John .
1. He kept crying all day long.
2. He kept on crying all day long.
(Is 'crying' a gerund or a present participle? What is the part of speech of 'on'?))
Actually a gerund IS a present participle! Both are said. Often, because English is a "directional" language (stand UP, sit DOWN, come IN, go OUT, etc.) you'll hear that sort of thing. The "on" in this sentence #2 goes with "to keep ON" or "to continue" for you could also say He continued crying.
The phrasal verb "keep on" (present) or "kept on" (past) is an idiom. It's almost impossible to explain its grammar!!
(first entry here)
John, the "keep on" expression is used with lots of following verbs.
keep on studying, working, eating, playing, etc. The implication is to "not quit", not give up.
P.S. You may or may not have heard the trucker's expression "Keep on keeping on!"