The Past, as refuge and burden
The Past, along with forgiveness, is one of two dominant themes in the play. At different parts, the Past plays different roles. On one hand the past is a burden. Mary speaks with a terrible fatalism, claiming that nothing they are can be helped: past sins and mistakes have fixed their present and future irrevocably. The past also takes the form of old hurts that have gone unforgiven. We hear the same arguments again and again in this play, as the Tyrone's dredge up the same old grievances. Letting go is impossible, and so the Tyrones are stuck.
The past also becomes a refuge, but not in a positive way. Mary uses an idealized recreation of her girlhood as escapist fantasy. As she sinks further and further into the fog of morphine, she relives her childhood at the Catholic girls' school. The past is used to escape dealing with the present.
Forgiveness
Forgiveness is the other pivotal theme of the play. Although old pains cannot be forgotten and the Tyrones are, in a way, a doomed family, Edmund is able to make peace with his past and move on to what we know will be a brilliant career. His ability to do so is based in part on his capacity for forgiveness and understanding. The four Tyrones are deeply, disturbingly human. They have their jealousies and hatreds; they also remain a family, with all the normal bonds of love, however troubled, that being a family entails. Unlike his brother, Edmund is able to forgive and understand all of the Tyrones, including himself.
Breakdown of communication
Breakdown of communication is a very apparent theme. We are forced to listen to the same arguments again and again because nothing ever gets resolved. The Tyrones fight, but often hide the most important feelings. There is a deep tendency towards denial in the family. Edmund tries to deny that his mother has returned to morphine. Mary denies Edmund's consumption. Often, avoidance is the strategy for dealing with problems.
Religion
Although Tyrone professes to keep his faith, his two sons have long since abandoned the Catholic religion. Tyrone's religion spills over into his taste in art. He considers Edmund's favorite writers to be morbid and degenerate. Mary's loss of faith also recurs as an issue. Although she still believes, she thinks she has fallen so far from God that she no longer has the right to pray.
Drug and alcohol abuse
Mary's morphine addiction is balanced by the men's alcoholism. Although the morphine is perhaps a more destructive drug, alcohol does its fair share of damage to the Tyrone men. It is Tyrone's great vice, and it has contributed to Mary's unhappiness. Drunkenness has been Jamie's response to life, and it is part of why he has failed so miserably. And Edmund's alcohol use has probably contributed to ruining his health.
Isolation
Although the four Tyrones live under the same roof this summer, there is a deep sense of isolation. Family meals, a central activity of family bonding, are absent from the play. Lunch happens between acts, and dinner falls apart as everyone in the family goes his separate way. Mary's isolation is particularly acute. She is isolated by her gender, as the only woman of the family, and by her morphine addiction, which pushes her farther and farther from reality.
