two questions about "passive voice"
1.You need to clean your room every day.
2.Three more man are needed to do the work every day.

I know i must use active voice in the first sentence ,and i must use passive voice in the second one. but i am not sure how to describe it to the kids.

Question

two questions about "passive voice"
1.You need to clean your room every day.
2.Three more man are needed to do the work every day.

I know i must use active voice in the first sentence  ,and i must use passive voice in the second one.  but i am not sure  how to describe it to the kids.

Answer

You better give examples of sentences in the active voice and how to turn them into passive voice. 
The first example you gave is an active one, but is not a good example because it doesn't have any object to serve as the subject of the passive voice . 

You better say instead: 
" I need you to clean the room every day." 
Then form the passive voice : 
" You are needed to clean the room every day. 

As for the second example Eliot gave you a good example of a sentence in the active voice 
"We need 3 more men to do the work." 
Then your sentence will be its passive voice version: 
"3 more men are needed to do the work." 

This way you are drawing a comparison between the 2 voices and showing the students how to form a sentence in the passive voice. 
You have to point out that the object of the active voice becomes the subject of the sentence in the passive voice.

Answer

2.Three more MEN are needed to do the work every day.

Why teach these two sentences together? They're not related!

Answer

For reference, the passive form of the first sentence is "Your room needs to be cleaned every day..."

Answer

For reference, the passive form of the first sentence is "Your room needs to be cleaned every day..."