1. We are at my cousin's school.

2. We are in my cousin's school.
[Can we use both prepositions?]

3. He plays his violin on stage.
4. He plays his violin on the stage.
[Are both correct? Which one is commonly used?]

5. I move in my seat.
6. I move on my seat.
[Which preposition do we have to use? Are both okay?]

All are acceptable.

1. Yes, both prepositions can be used in this context. However, there is a slight difference in meaning between them. "We are at my cousin's school" implies that we are physically present in the vicinity of the school but might not necessarily be inside the building. "We are in my cousin's school" suggests that we are inside the building.

2. Both "He plays his violin on stage" and "He plays his violin on the stage" are grammatically correct. However, the commonly used phrase is "He plays his violin on stage." This implies that he is performing in a specific area designated for performances and does not require any further elaboration on which stage it is.

3. The phrase "I move in my seat" is the most commonly used and natural-sounding option. It suggests that you are shifting your position while sitting down. On the other hand, "I move on my seat" is not commonly used and may sound awkward. It could imply a more significant movement, such as standing up or changing seats.

1. Yes, you can use both prepositions in these sentences interchangeably. Both "at" and "in" are commonly used to indicate being physically present within a location.

2. Similarly, both "on stage" and "on the stage" are correct and can be used interchangeably. However, "on stage" is more commonly used to refer to performing or being present on a theatrical stage.

3. Both sentences 5 and 6 are grammatically correct, but they convey slightly different meanings.

- "I move in my seat" suggests that you are moving while remaining seated, possibly shifting your position or adjusting your posture.
- "I move on my seat" suggests that you are moving while actively interacting with or using your seat (e.g., sliding along the seat, bouncing on the seat).

The appropriate preposition to use would depend on the specific context and the intended meaning you want to convey.