Of these juxtaposed ideas, the one most obviously central to the play is that of fantasy versus reality. Gallimard lives in a make-believe world by carrying on his affair with Song and apparently not guessing that Song is actually a man. And Gallimard's wife is being deceived on two levels: first in the more conventional fact of Gallimard having a lover, and second in not understanding Gallimard's "true" nature.
What Song himself must perceive about the situation is perhaps more complex. He is being "used" by his government to carry out this bizarre deception, but to us, the reader and audience, it's unknown to what extent he is merely play acting and not actually expressing some level of need that is genuine in his affair with Gallimard. The dichotomy of "past" and "present" is a more subtle and ambiguous issue. We first see Gallimard as an old man looking back on his sorry history and in effect re-creating it for us.
The shifts in time give the play a dreamlike (or nightmarish) quality, but also raise the question of how much is "real" and how much an exaggeration or transformation by Gallimard of events that have led to his downfall. Is Gallimard the ultimate unreliable narrator? The explicit references to the opera Madama Butterfly show Hwang's play to be a kind of ghost of a work which has become a cultural trope for east-west relations and the exploitation inherent in them. But in this case, the dynamic between east and west has been reversed. Gallimard's actions in his affair with Song represent the desire to re-enact the old dynamic between different cultures, but the result for him is a catastrophe that makes the terms "frustration" and "loss" seem like vast understatements.