We use the passive voice to give emphasis to something that has happened recently or in the past.
That's why we find it in the news headings ( for example :" A bank was robbed last night") or when we talk about inventions and discoveries (" The radio was invented by Marconi).
2. Grammar: how do we make the passive voice?
We have a subject "the radio", a verb "was invented" and the agent "by Marconi".
The agent is the subject of the active form : "Marconi invented the radio".
It is not always necessary to express the agent. In the above mentioned example " A bank was robbed last night" we don't find any agent because what is important here is what happened to the bank, not who robbed the bank.
3. From active to passive
Let's go back to Marconi to understand how to convert an active sentence into a passive one.
In the active sentence "Marconi invented the radio" we have: Subject= "Marconi, a Verb "invented" (simple past) and an Object "the radio".
The active voice structure is then SVO.
When we convert the sentence into a passive voice the Object "radio" becomes the Subject while the Subject "Marconi" becomes the Agent "by Marconi".
"The radio was invented by Marconi".
4.How to convert the verb?
The verb in the passive voice is "to be+past participle".
In "Marconi invented the radio" we have a regular simple past "invented" and a singular subject "Marconi". So what you really have to focus on is to conjugate the verb TO BE according to the person(I,you, he, she, it, we, you, they) and the tense ( here we have a simple past).
The simple past of the verb to be for the subject "Marconi" is WAS. To WAS we add the past participle of the verb "invent" that is "invented" so we have "The radio was invented by Marconi"
Here you will find more details about how to convert active sentences into passive and some online exercises, too.