Understanding Active Voice
To convert the sentence "Wherefrom had this mail been sent by you?" into active voice, we need to identify the subject and make it the doer of the action.

Original Sentence Structure
- **Passive Voice**: The structure is in passive voice where "this mail" is the subject receiving the action.
- **Auxiliary Verb**: "had been sent" indicates a past action.
- **Agent**: "you" is the doer of the action but is placed at the end of the sentence.

Steps to Change to Active Voice
1. **Identify the Subject**: In this case, "you" is the subject who performed the action.
2. **Rearrange the Sentence**: Place the subject at the beginning of the sentence and make it active.
3. **Change Verb Form**: Modify the verb to reflect the active voice.

Active Voice Conversion
- **Active Voice**: "Where did you send this mail from?"

Key Changes Explained
- **Subject Position**: "You" is now at the forefront, making it clear who is performing the action.
- **Verb Adjustment**: The verb "send" is used in its base form, which is common in active voice structures.
- **Clarity**: The active voice sentence is more direct and easier to understand, enhancing clarity.

Conclusion
Converting sentences from passive to active voice not only clarifies the subject's role but also streamlines communication, making it more engaging and straightforward. Understanding these transformations is crucial for effective writing, especially in competitive exams like SSC CGL.