this passage is all one thought; and it is about giving feedback - or coaching.
in a word, this passage is about tact.
definition of tact is: sensitivity in dealing with others or with difficult issues.
if someone makes a mistake (in this case a grammatical mistake), don't be that "know-it-all" and immediately jump all over the person.
think of Hermione and how she somewhat arrogantly corrects Ron: https://www.youtube.com/watch?v=fx2zLgdkJpk