Finished can be used as a normal past participle in perfect verb forms (e.g. I’ve finished). It can also be used as an adjective after be, meaning “ready” (e.g. I’m nearly finished). There is not much difference of meaning between I etc am finished (in this sense) and I etc have finished. The adjective construction (be finished) is common in an informal style.
- How soon will you be/have finished, dear?
- Hang on – I’m/I’ve nearly finished.
- I went to get the car from the garage, but they weren’t/hadn’t finished.