John and Elizabeth's relationship is absolutely rocky...maybe it could even be described as sitting on shifting sand. However, they were once very strong and madly in love which comes through especially at the end when they once again are a force with which to be reckoned. John has wandered and has been tempted by another woman. Elizabeth is understandably hurt by this, and finds it hard to trust him again. They are not unlike most people who have different friends and interests but still love each other very much. People change and grow as they get older...some relationships strengthen as a result and others become brittle and break apart. For John and Elizabeth the first it true.
So, after thinking about what happens in their relationship and how these changes become evident through actions, thoughts, and speech in the play, you could begin it something like this:
As long as people have lived on the earth, they have suffered tragedy. For some, the tragedy makes them stronger; others fall apart as a result, but all of them are effected. The tragedy John and Elizabeth Proctor suffer serves to strengthen their relationship and they come through it stronger and wiser people.