Active to passive (step by step)1. Find the direct object in the active sentence.
John has bought many houses.
2. Put DO at beginning of new sentence
Many houses ...
3. Find the verbs
John bought many houses.
4. Determine the tense of the verb phrase from the active sentence.
bought = past simple
5. Convert according to the tenses of the passive
bought --> was/were bought
6. Rewrite the sentence.
Many houses were bought (by John).