Complex sentence: They can appear in periphrastic verb catenae, in which case they help form the main predicate of a clause (as illustrated with the trees below), or they can appear essentially as an adjective modifying a noun.
Split into simpler sentences: They can appear in periphrastic verb catenae, when they help form the main predicate of a clause (as illustrated with the trees below). Also, they can appear essentially as an adjective modifying a noun.