A good answer would mention their communication failure that seems to be the result of two very different outlooks on healing. Beth wants to move on with life rather than constantly dwelling on the past. Calvin, on the other hand, wants to heal by talking through the past and seeing how everyone is feeling, particularly his son. On this issue, Beth and Calvin can never agree to disagree because they feel that the stakes are too high. Also, Beth as a character seems resistant to change. Towards the end of the novel, she tells Calvin in a straightforward fashion that she has not changed at all; he is the only one who has changed. And she dislikes the way in which he has changed. It should be noted that these problems seem insurmountable, which is what leads to their separation at the end of the novel.
Over the course of the novel, we see that Calvin and Conrad come together. They begin the novel with a very awkward relationship. Calvin insists on always worrying about his son, but he also wants to seem like he is not breathing down his neck all the time. Conrad, by contrast, does not want his father to worry about him. Throughout the novel, Beth accurately points out that Calvin will never criticize Conrad for anything at all. Beth thinks that Conrad has Calvin wrapped around his finger. It is only at the end of the novel when Beth has gone and Calvin finally does criticize his son that they are able to come together in a real relationship. The novel ends on a very optimistic note with regards to the Calvin-Conrad relationship.
A good answer would discuss their role as examples for how the other characters