In German, an entire relative clause such as "the man who is reading" can be compressed into a single adjective placed directly before the noun, using one of two verb-derived forms: Partizip I and Partizip II. This corresponds functionally to the active and passive participle: Partizip I behaves like an active participle (the reading man, the sleeping child, the crying woman), while Partizip II behaves like a passive one (the read book, the opened door, the cooked meal). The basic idea — a verb form doing an adjective's job — is familiar from many languages; what has to be learned from scratch is the specific German morphology and the exact conditions each form requires.
Partizip I is formed by adding the suffix -end to the infinitive stem, with no other change: lesen becomes lesend, schlafen becomes schlafend. It always describes an ongoing, active process — the noun it modifies is the one actually performing the action at that moment: der lesende Mann is the man who is reading right now, not a man who read or will read.
Partizip II is the same past-participle form used in the Perfekt tense (gelesen, geöffnet, gekocht). Used as an adjective, it usually signals a completed action, and with most transitive verbs it carries a passive sense: das gelesene Buch is the book that was read (that has been read), not the book that reads. With intransitive verbs of motion or change of state that form their Perfekt with sein — fallen, ankommen, aufwachen — Partizip II carries no passive meaning at all, only an active, completed one: der gefallene Baum is the tree that has fallen, not a tree that someone felled.
Grammatically, both forms behave exactly like any ordinary attributive adjective: they take the standard adjective-ending pattern according to the noun's gender, number, and case, and according to whether a definite article, indefinite article, or no article precedes them — following the same rules as klein or groß.
The most common mistake is assuming Partizip II always signals a passive meaning, leading learners to misread der gefallene Mann as "the man who was made to fall" instead of "the man who fell," forgetting that intransitive verbs have no passive at all. A second frequent error is forgetting the adjective ending after the participle, producing mit dem lesend Mann instead of the correct mit dem lesenden Mann.