Does a passive sentence include the word by?

Not necessarily. A passive sentence doesn't always have to include the word "by".

Here's why:

* Passive voice focuses on the action being done, not the doer. The sentence structure emphasizes the object of the action.

* "By" is used to introduce the agent (the doer) of the action. But, the agent isn't always important or known.

Examples:

* Passive without "by": The cake was eaten. (We don't know who ate it)

* Passive with "by": The cake was eaten by John. (We know who ate it)

So, while "by" can be present in passive sentences, it's not a defining characteristic. The key element is that the object of the action becomes the subject of the sentence.