Causative get/ have sth doing

• Most of the time, we do not show who does/did an action (the 'agent') in a passive or causative sentence. When we do want to refer to the agent, we use by.
✓ We should get the car looked at by a professional.

• When we want to refer to materials or instruments used in a passive or causative sentence, we use with.
✓ The engine is started with a special electronic card instead of a key.

• We use other prepositions after some past participles that are used like adjectives.
✓ I am frightened of driving on motorways.
✓ My car is covered in dirt.