Special note, blow is a change of state verb:
The storm has blown the tree down. (Active)

The tree has been blown down by. (Passive)

The tree has blown down. (No Agent)

Active: The storm has blown the tree down.
Passive: It has been blown down <by the storm>.
Ergative: It has blown down. <no agent>


1. It is stormy. The roof has blown off! <ergative>

2. The tree was lying across the road. It has been blown down in the storm, by the strong winds. <passive, and the subjects are the same , which is why passive voice is used.>