BY: We only use “by + agent” when it is important to say who or what is responsible for something.
- The radio was invented by Marconi.
- A number of trees were blown down by the strong winds.
NOTE: We sometimes use the subject of an active sentence (Marconi, the strong winds) as ‘the agent’ in a passive sentence. When this happens, we use “by” to introduce the agent in the passive.
WITH: We use “with” to talk about an instrument which is used by the agent to do something.
- I was hit with an umbrella.
- The World Trade Center was crashed with two aircraft.
NOTE: We also use “with” to talk about materials or ingredients.
- The room was filled with smoke.
- Irish coffee is made with whiskey.