Grammar
Here's an analysis of the sentences you provided, focusing on tense:
1. The bell had rung before we entered the class.
Past Perfect: "had rung" - This describes an action completed before another action in the past.
Simple Past: "entered" - This describes an action that happened in the past.
Meaning: The bell rang sometime before we entered the classroom.
2. I went for a walk after I had eaten my dinner.
Past Perfect: "had eaten" - This describes an action completed before another action in the past.
Simple Past: "went" - This describes an action that happened in the past.
Meaning: First, I finished eating my dinner, and then I went for a walk.
3. Had the train left when she arrived?
Past Perfect: "Had...left" - This is in the form of a question, asking if an action was completed before another action in the past.
Simple Past: "arrived" - This describes an action that happened in the past.
Meaning: Did the train leave before she arrived?