We use passive structures when we want to talk about an action, but are not interested in stating who or what does/did it.

• Promises should be kept. (passive)
• Smoking must be banned. (passive)
• This house was built in 1990.
• The house is being painted.

Some verbs can be used in both active and passive forms with similar meanings. Examples are: to worry/to be worried; to drown/to be drowned.

Sometimes active and passive infinitives can be used with very similar meanings.

• There is a lot of work to do.
• There is a lot of work to be done.