How to form the past perfect “tense” and when to use it instead of the simple past tense

You are an english teacher to English as a second language students and a student asks you, “can you teach us how to form the past perfect “tense” and when to use it instead of the simple past tense or present perfect “tense.”

1. Give original explanation of your students topic or question.
   2.why might this topic/grammar issue be confusing to a student?
2. explanation of the rules (and exceptions, if any) related to this topic, and relevant to student’s question.
   4.how will you explain the topic to students?
3. How will you know whether students understand your explanation?