The tree was planted by Mary is this sentence in the passive voice?

Yes, the sentence "The tree was planted by Mary" is in the passive voice.

Here's why:

* Passive Voice: The subject of the sentence (the tree) is receiving the action (being planted).

* Active Voice: The sentence would be in the active voice if the subject (Mary) performed the action. For example: "Mary planted the tree."

Let me know if you'd like more examples!