Complex sentence: The past participle is used generally as an adjective meaning a finished action, and it is variable in gender and number in these uses; and also it is used to form the compound tenses (as in English) in which it is indeclinable.
Split into simpler sentences: The past participle is used generally as an adjective referring to a finished action, in which case its ending changes according to gender and number. At other times is used to form compound tenses (as in English), in which case it is indeclinable.