Three conditions for Monitor use:
Time: In order to think about and use conscious rules effectively, a second language learner needs to have time.
Focus on form: The performer must also be focused on form, or thinking about correctness to get his message across in an understandable way to the listener(s).
Know the rule: This is very formidable requirement. If rules are not known ( learner does not know the rule), the Monitor will not be helpful or used.