① When we are employed in reading a great and good author, we ought to consider ourselves as searching after treasures, which, if well and regularly laid up in the mind, will be of use to us on various occasions in our lives.

② Children, who learn quickly, should start school as early as possible.

≠ Children who learn quickly should start school as early as possible.

③ It is a fine opportunity, which I would seize if I were not engaged.

④ There was a letter from Mary, which he insisted upon opening.