1. It is not good for the painting.

2. It is not good for the paintings.

(Are both OK? Which one is frequently used when we mention many pictures in a museum.)

3. How was the first day of school?
It was good. So many of us changed.

(In the dialogue, what is the meaning of 'So'?)

Both 1 and 2 are correct. #2 is used when referring to more than one painting.

In 3, the word "So" is modifying "many" -- telling to what degree. It's simply used for emphasis: Not just a few of us changed; MANY of us changed!

2. is correct, paintings indicate more than one.

3. so in that context means to a great degree, so many then means great many. The same meaning would have been conveyed by "A great many of us changed."

1. Both sentences are grammatically correct, but they differ in meaning.

- "It is not good for the painting" refers to a singular painting, indicating that something is not beneficial for that specific artwork.
- "It is not good for the paintings" refers to multiple paintings, suggesting that something is not favorable for the whole collection or various artworks in the museum.

That being said, when mentioning many pictures in a museum, the phrase "It is not good for the paintings" is more frequently used.

2. In the given dialogue, "So" is used as a conjunction to express the result or consequence of the preceding statement. In this context, it means that because the first day of school was good, many of the speakers and their peers experienced change. It implies a cause-and-effect relationship between the positive experience of the first day and the subsequent transformations or modifications that occurred.