I agree with Bert, Mounce is really good at teaching you the English before getting into the Greek. And it's really hard to try to understand what a Greek participle is until you understand what an English participle is. Keep in mind though, the participle in Greek is not identical to its English counterpart, as is the case in most things in language study, somewhat like the definite article in Greek which is not identical to "the" in English. Speaking abstractly, the participle's basic function is as a verbal adjective (compare with an infinitive which is a verbal noun). It modifies a noun or phrase by use of a verbal form.