1. “by” and the passive : In the passive voice, “by” indicates WHO is doing the action. examples:
Sarah wrote the book. (active)
The book was written by Sarah. (passive)

2. “by” + reflexive pronoun : by + myself / yourself
examples: I enjoy reading by myself.

3. “by” + -ING verb : This structure describes how to do something. It gives us more information about how to achieve a particular result.
examples: How can I turn on the radio?

4. “by” + time expression : The meaning of this structure is not later than; before or at a particular time . We use this structure for deadlines. A deadline is the time before which something must be done.
examples: Guests must vacate their hotel rooms by 11 am.