"In English grammar, participles ....."
"In English grammar, the past participle refers to an action that was started and completed entirely in the past."
The past one of those - definite
English grammar aspects confuse many.
The gerund arises form a need and its syntax form older languages ....
Which grammar aspect?
but you could also say
"In English grammar, a past participle refers to an action that was started and completed entirely in the past."
Here it means, not the general concept fo a past participle but any specific one, an example one.
The definite article is difficult to explain. I'm not sure that it has derivable rules for every case. It may be that with enough use of "correct" cases, it just becomes natural. :)