Question

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'?))

Answer 1

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.

Sra

Answer 2

The phrasal verb "keep on" (present) or "kept on" (past) is an idiom. It's almost impossible to explain its grammar!!
http://www.eslcafe.com/pv/pv-k.html
(first entry here)

Answer 3

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.

Answer 4

P.S. You may or may not have heard the trucker's expression "Keep on keeping on!"

Sra