English: Active And Passive Voice

How you understand active and passive verbs?

We use an active verb to say what the subject does.

Example:

1) My friend built this house in 2000.

2) My company employs two hundred people.

We use a passive verb to say what happens to the subject.

Example:

1) This house was built by my friend in 2000.

2) Two hundred people are employed by my company.

Note:

Use of "by" is not necessary to use a passive voice.

A passive sentence can be written without by. 

Example: 

The thief was caught.

In this sentence who did the job is not important or we say the "agent" is not important or unknown. So agent is not mentioned. For the passive voice we use a helping verb using a form of be (is/was) + the past participle of the verb (done/cleaned/written/seen).