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.
Sra
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)
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!"
Sra
In both sentences, 'crying' is a gerund. A gerund is a verb form that functions as a noun. In this case, 'crying' is the subject complement of the sentence, referring to the action of crying.
The word 'on' in the second sentence is a particle in the phrasal verb 'kept on.' In this context, 'on' serves to intensify the action of crying. The phrasal verb 'kept on' means to continue doing something. So, 'kept on crying' means that he continued to cry without stopping throughout the whole day.
To identify whether 'crying' is a gerund or a present participle, you can look for clues. In this case, the use of 'kept' suggests a continuous action, which points towards 'crying' being a gerund since gerunds are used to describe ongoing activities.