Sentence-Structure of Active Voice:
Sentence-Structure of Active Voice is same as a simple sentence has.
Subject + Verb + Preposition + Object + Adverb
(Preposition and Adverb are not necessary but if they exist, their place will be the one as described)

Sentence-Structure of Passive Voice:
Now see the difference of structure given below:
Object + Helping Verb + 3rd Form of Verb + Preposition + By + Subject + Adverb
This structure is followed by the Passive Voice. Once you learn it, you can convert every active voice into passive voice.
While converting Active Voice into Passive Voice, two changes are must i.e. Helping Verb and 3rd Form.

Role of Helping Verb in Passive Voice:
Actually the Helping Verb to be used in Passive Voice is only one i.e. “Be”. Its forms are used with respect to Principle (Main) Verb of Active Voice.
Following table explains which ones are the forms of “Be” with respect to different tenses.

Some Rules for Conversion:

You have to focus on the use of Helping Verb in Passive Voice keeping in mind the Main Verb in Active Voice.
1– If the form of verb is 1st in Active Voice, then use first forms of “Be” i.e. is (For Singulars), are (For Plurals), and am (For I”)
Example:
I pluck a flower.
A flower is plucked by me.

2– If 2nd form of verb is there in Active Voice, then use 2nd form of “Be” i.e. Was (For Singulars) and Were (For Plurals)
Example:
I plucked a flower.
A flower was plucked by me.

3– If the form of verb is 1st in Active Voice and “will or shall” is used with it, then use will (with He, she, it, they and any name) and shall (with First Person i.e. I, we) and then apply first forms of “Be” same as it is means “Be”.
Example:
I shall pluck a flower.
A flower will be picked by me.

4– There is always 3rd form in all the Perfect Tenses, so add 3rd form of “Be” i.e. “Been” and use has for singulars, have for plurals and had for all in past perfect in their Passive Voice.
Examples:
I have plucked a flower.
A flower has been plucked by me.
I had plucked a flower.
A flower had been plucked by me.
I had plucked a flower.
I shall have plucked a flower.
A flower will have been plucked by me.

5– When you find 1st form with “ing” in Active Voice, simply add “ing” with “Be” i.e. Being.
Examples:
I am plucking a flower.
A flower is being plucked by me.
I was plucking a flower.
A flower was being plucked by me.

6– Now look at the following sentence.
I shall be plucking a flower.
A flower will be being plucked by me.
Here the structure goes wrong because “Be” and “Being” can not be used together, that’s why there is no passive voice of Future Continuous Tense. Same is the case with all Perfect Continuous Tenses.

Imperative Sentences:

As you studied earlier, Imperative Sentences include advice, request, order or prohibition. The structure of these sentences is somewhat different, so the Passive Voice will also be different. Observe the following Sentence-Structure:

Active Voice for Imperatives:
Verb + Preposition + Object + Adverb

Passive Voice for Imperatives:
Let + Object + be + 3rd Form + Preposition + Adverb
There is no subject in Imperative sentences and remember, if the verb comes first in a sentence, then always 1st form of verb is used. So in Passive Voice, “By” and “Subject” are omitted because in its Active Voice, there was no action doer.
Example:
Knock at the door quickly.
Let the door be knocked at, quickly.