How to disable sub-form fields?

How to disable sub-form fields?

For each line in my sub-form, I have various fields that I don't want the user to be able to edit. There are also a few fields that I want to be kept hidden but I can't seem to figure out how to do that either. I've created a new workflow for adding a new row and in it I'm saying the following. 

  1. disable row.Item_Price;
  2. hide row.ID;
For both of these statements, I keep getting an error message that says "Invalid task in sub-form action." Does anyone know why it says this and how I can hide and disable a sub-form field?