In most cases, the subject of an active verb (the agent) is not mentioned in the corresponding passive sentence. If it does have to be mentioned, we usually use an expression with by.

• They gave me a warm welcome. (active)
• I was given a warm welcome by them. (passive)
• Children love toys. (active)
• Toys are loved by children. (passive)
• They built this house. (active)
• This house was built by them.
• Her attitude shocked me.
• I was shocked by her attitude.

It should be noted that by is not the only word with which the agent can be introduced. After the past participles of some 'stative’ verbs (verbs which refer to states, not actions) other prepositions can be used instead of by.

• The state of his health worries me. (active)
• I am worried about the state of his health. (passive)
• Snakes scare me. (active)
• I am scared of snakes. (passive)

With is used when we talk about an instrument which is used by an agent to do an action.

• He killed the snake with a stick. (active)
• The snake was killed (by him) with a stick. (passive)