Couple of definitions:
The teacher is frustrating.
Which is this picture?
The kitten is frightened. => The frightened kitten hid behind the jar.
One last note: not only can progressive verbs act as adjectives, they can also act as nouns! When -ing words are used as nouns they aren't called progressive. Anyone know what they are called (yes, another topic for another day)?
The versatility of language is amazing!