(NOT a teacher): INDICATIVE (a fact)/ SUBJUNCTIVE (a wish in speaker's mind). (1)Tom: Mona expresses her opinions too much. I'm going to tell her to stop. You: Don't do that. It is important that she expresses her opinions. It's good for her mental health. (2) Tom: Mona never expresses her opinions. I'm going to tell her to start talking more. You: Don't do that. It's important that she does not express her opinions. She does not want to offend anyone. (3) Tom: Mona never expresses her opinions. I think she should. You: You're right. She is too quiet. She is keeping her feelings pent up. I agree that it is important that she EXPRESS her opinions more. (I hope she will, but who knows.) (3) Tom: Mona says that she plans to express her opinion about the secret we three share. You: Oh, no! It is important/It is my wish/ It is my demand that she NOT EXPRESS her opinion. If she does, all three of us will be in big trouble.
Interested in Language