Subject Verb Agreement in Active and Passive Voice

The passive forms of a verb are created by combining a form of the "to be verb" with the past participle of the main verb. Other helping verbs are also sometimes present: The passive can be used, also, in various tenses.
 Let's take a look at the passive forms of "design.
 

Tense	Subject	Auxiliary		Past  Participle
		Singular	Plural
Present	The car/cars	is	are	designed.
Present perfect	The car/cars	has been	have been	designed.
Past	The car/cars	was	were	designed.
Past perfect	The car/cars	had been	had been	designed.
Future	The car/cars	will be	will be	designed.
Future perfect	The car/cars	will have been	will have been	designed.
Present progressive	The car/cars	is being	are being	designed.
Past progressive	The car/cars	was being	were being	designed.

 
Note the use of subject verb agreement in the passive.