Introduction:
Since and for are prepositions used to indicate the duration of an action or event. They are commonly used with the present perfect tense to express the length of time an action has been happening or a state has been in existence.

Use of since:
1. Time reference: Since is used to specify the starting point of an action or event. It is followed by a specific point in time, such as a particular date, year, or time.
- Example: She has been living in London since 2010.

2. Starting point in the past: Since can also be used to indicate the starting point of an action or event that began in the past and continues in the present.
- Example: I have known him since we were children.

3. Cause or reason: Since can be used to express the cause or reason for something.
- Example: Since it's raining, let's stay indoors.

Use of for:
1. Duration of time: For is used to indicate the length of time an action or state has continued. It is followed by a specific period of time, such as hours, days, weeks, months, or years.
- Example: They have been studying for three hours.

2. Future events: For can also be used to talk about future events that will continue for a certain period of time.
- Example: I will be on vacation for two weeks.

3. Emphasizing duration: For can be used to emphasize the duration of an action or event.
- Example: He worked tirelessly for years to achieve his goals.

Key Differences:
- Since is used to indicate the starting point of an action or event, while for is used to indicate the duration of time.
- Since is followed by a specific point in time, whereas for is followed by a specific period of time.
- Since is commonly used with the present perfect tense, while for can be used with various tenses.
- Since can also express cause or reason, which is not the case with for.

Conclusion:
Since and for are prepositions used to express the duration of an action or event. Since indicates the starting point or cause, while for indicates the duration. Understanding the correct usage of since and for is important for maintaining grammatical accuracy in English communication.