Making active sentences passive
A man is reading a book.
In this sentence 'a man' is the subject of the sentence and 'a book' is the direct object.
To change this active sentence into the passive you take the direct object 'a book' and change it into the subject and 'a man' becomes the agent, not the direct object of the sentence. So you need to add the preposition 'by' which can signify the agent of the action of the verb, which in this case is 'is reading'.
So we get the sentence:
A book is (being) read by a man.
This sentence is a simple passive present tense sentence. The first sentence is in a simple active present tense.