In Ghosts, how does Ibsen reveal past events that are necessary to understand the present?
Ghosts' main theme is how deception is likely to destroy the best and the worst of people. The effect of this can be seen right from the beginning with Regina's speculation that her father, Jacob Engstand, despite his protestations, is perhaps not her real father.
Regina a housemaid, is prepared to perform her duties and, ironically much like her mother, become mistress to, in her case, Oswald, if it means that she may be able to acquire wealth by marrying him. It is blatantly obvious when, upon discovering that Oswald is, in fact, her half-brother, she takes steps to leave the house immediately. Perhaps, had she known as a child, she could have developed a healthy relationship with Oswald and support him now in his illness. Upon seeing that there is no profit in it, she will take any opportunity to seek her fortune elsewhere despite what appeared to be genuine affection at first.
Each character in Ghosts has actually perpetuated the deceit as they all try to hide aspects of their lives; whether from a misguided belief that it's for the best, as with Mrs Alving, or from a selfish perspective, such as Oswald's and Regina's or even from the pastor's viewpoint where appearances are everything and it is acceptable to hide the truth in order to protect others - and especially himself!
The pastor is so easily led; "your reverence" is impressed by Jacob's attempts to make a "sailor's home." He knows that it is unlikely that Jacob is the "guardian angel" he proclaims to be but it is easier and less offensive to believe that his intentions are good. He is so confused and desperate to believe in the goodness of all (because it's just less complicated), that he even effectively takes the blame for setting fire to the orphanage- a most improbable cause.
In understanding where each of these characters are coming from, the story becomes believable, possible and, in Ibsen's day, very likely as there were those people always manipulating the truth for their own ends and there was a belief that ‘‘The sins of the fathers are visited on the children’’ bringing a stark truth to the current circumstances in which the characters find themselves.