The preposition "by" is quite versatile in English and can be used in various contexts with different meanings. Here are the main uses:

1. Indicating Means or Method:
"By" is used to indicate how something is done or achieved.
- **Example**: "She traveled by train." (indicating the mode of transportation)
- **Example**: "He won by practicing every day." (indicating the method of achieving something)

2. Indicating Time:
"By" can specify a deadline or a point in time by which something will happen or be completed.
- **Example**: "Please finish the report by Monday." (indicating the latest time to complete the task)

3. Indicating Proximity or Location:
"By" is used to indicate that something is near or next to something else.
- **Example**: "The house is by the lake." (indicating proximity)

4. Indicating Authorship or Agency:
"By" is used to indicate who created something or who is responsible for an action.
- **Example**: "This book is written by J.K. Rowling." (indicating authorship)
- **Example**: "The cake was baked by my sister." (indicating who performed the action)

5. Indicating a Passive Agent:
In passive constructions, "by" introduces the agent performing the action.
- **Example**: "The song was sung by the choir." (indicating who performed the action in a passive sentence)

6. Indicating Measurement or Units:
"By" is used in expressions of measurement or units.
- **Example**: "They sell apples by the kilogram." (indicating the unit of measure)
- **Example**: "The room is 10 feet by 12 feet." (indicating dimensions)

7. Indicating Support or Opposition:
"By" can be used to show support or opposition to something.
- **Example**: "She stood by her friend during tough times." (indicating support)

8. Indicating a Standard or Rule:
"By" can indicate conformity to a rule, standard, or principle.
- **Example**: "By law, you must wear a seatbelt." (indicating conformity to a rule)

Understanding the context in which "by" is used is crucial, as its meaning can change significantly based on how it is applied in a sentence.