Student or Learner
If I say, " the profit margins are thin which is carved up by others"
Do I use "which is carved up" or "which are carved up"?
The sentence is incorrectly constructed. You should write something like:
"The thin profit margin is carved up by others" or "The thin profit margins are carved up by others". Alternatively, you could add more detail and move towards the "which" structure:
".. created a thin profit margin, which is carved up by others."