The main problem in the book That Was Then, This Is Now is the growing rift between Bryon and Mark, as Bryon begins to "grow up" while Mark does not.  The two boys have been raised together since Mark was left homeless after the violent death of his parents, and until this time, they have been as close as brothers.  Both boys have been scrappers, enjoying hustling at pool and frequent fights with their neighborhood enemies.  Mark especially has led a charmed existence during the past few years, seemingly being able to get away with just about anything.

Now that they are sixteen, however, Bryon has started to think more deeply about life and responsibility.  When their friend Charlie is killed while trying to defend them after they hustle some guys at the pool hall, Bryon is overcome with a sense of guilt, and when Bryon himself is brutally beaten by the Shephards, he refuses to allow Mark to retaliate for him, telling him, "I don't want to keep this up, this getting-even jazz.  It's stupid and I'm sick of it and it keeps going in circles" (Chapter 8). 

Mark, on his part, would prefer to continue living a carefree, unexamined life.  Unhappy at the changes that he sees Bryon going through, he tells him, "You can't keep trying to figure out why things happen, man...You gotta just take things as they come, and quit trying to reason them out" (Chapter 7).

The growing rift between Bryon and Mark is never really solved, which illustrates the central theme of the story.  The process of growing up is a difficult thing, because one must leave behind the certainties of childhood and face the complicated and oftentimes unsolveable dilemmas of adulthood.  The story ends after Bryon turns Mark in to the police for selling drugs, and there is no happy resolution.  Mark is chillingly embittered by what he considers to be Bryon's betrayal and seems headed on a downhill spiral of ever increasing antisocial behavior.  Bryon, on his part, is bewildered and consumed by guilt, "too mixed up to really care".  In the last line of the story, he can only say, 'I wish I was a kid again, when I had all the answers" (Chapter 11).

How is the main problem solved in That Was Then, This Is Now?

The primary conflict of Susan E. Hinton's novel, That Was Then, This Is Now, eventually comes down to the changes that are occurring between the inseparable friends, Bryon Douglas and his foster brother, Mark. Two events cause the irreparable split between the two lifelong pals: After Mark gets Angela drunk and cuts off her hair, her two brothers--Tim and Curly Shepard (of The Outsiders)--confront Bryon and beat him senseless. His refusal to tell the Shepards that it was actually Mark who is responsible for their sister's unwanted haircut causes Mark to feel he now owes Bryon a debt that he can no longer pay.

Later, in Chapter 10, while searching for a cigarette, Bryon discovers Mark's secret drug stash and realizes that he has been dealing drugs in the hippie house where M&M has been staying. In a moment of decisiveness, Bryon calls the police and reports Mark. Mark is arrested and taken to jail--forever ending the friendship between the two boys.