Parts of speech include verbs as vital elements. Without verbs there can be no complete sentence. Verbs signify action and when action is involved subjects and objects must enter the scene. Also when verbs signify action then tense is not far away.
When you want to emphasise an action you could do so in either the active voice or the passive voice. In the active voice the SUBJECT PERFORMS the action and in the passive voice the SUBJECT of the verb is ACTED UPON. To give an example, consider the sentence, "SAURAV SINGS A SONG". In this sentence SAURAV is the subject and he performs the action. Now consider the sentence, "A SONG IS SUNG BY SAURAV". In this sentence the subject of the verb is ACTED UPON. Thus, VOICE shows whether the SUBJECT of the VERB IS ACTED UPON or the SUBJECT of the VERB ACTS.
Since the meaning of the sentence remains the same in both active and passive voices, there are rules to be followed when changing the sentence from an active voice to a passive voice. The change is incorporated through changing the verb. The object of the active verb becomes the subject of the passive verb. The subject of the active verb is made the object by use of the preposition ‘by’. The main verb is changed by adding an appropriate form of the verb ‘be.’ The tense of the verb does not change. Initially, it is a bit confusing but when you take the quiz that follows you will begin to get a grasp of passive and active voices.