posted by John .
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."