How do you change sentences with verbs in the active voice into the passive voice?

Answer 1

You reverse the subject and object in a sentence. Thus:

The dog ate the bone becomes - the bone was eaten by the dog.

ALSO:

It is not always necessary to add who or what does the action in a passive sentence. One of the main uses of passive is when, who or what does the action (the agent) does not matter or is not known eg:

Tea is picked and is taken to the drying sheds. There it is spread out to dry and then packed into large boxes for transport to the packing facilities.

Answer 2

To change a sentence from active voice to passive voice, the object of the active sentence becomes the subject of the passive sentence, the verb is changed to a form of "to be" plus the past participle of the main verb, and the subject of the active sentence is added with "by" before it as the object of the passive sentence (if needed). For example, "The dog chased the cat (active)" becomes "The cat was chased by the dog (passive)."