There are events that stand out on the first act of Lady Windermere's Fan.
The first is when Lady Windermere's butler asks her if she will be available for visits and informs her that Lord Darlington will be stopping by the house. Lady Windermere reacted with hesitation at this information, leaving the audience wondering what could be happening.
This may seem unimportant but, as Lord Darlington enters and begins his conversation with Lady Windermere, we learn that the night before he had been making uncomfortable comments and paying excessive compliments to her. The way in which Lady Windermere tells him that he should not do that again, makes the audience sense that there is a subtle but mutual attraction between Lady Windermere and Lord Darlington. However, Lady Windermere is quite clear in her duties to her husband, and in her dignity as a lady.
Act I also shows when the Duchess of Berwick visits Lady Windermere and discloses the rumor that is going all over London: That Lord Windermere is financially supporting a woman known as Mrs. Erlynne.
These two situations are the factors that set the plot in motion: First, when Lady Windermere confronts her husband about the rumors is when we realize that Lord Windermere is secretly trying to get Mrs. Erlynne back in society so that she can disclose that she is Lady Windermere's mother.
Second, when Lady Windermere loses her nerve out of the anger that her husband has caused her she runs straight to Lord Darlington for comfort and, perhaps, for a romantic meeting with Darlington to get back at her husband. This directly contradicts her previous behavior in which she showed signs of annoyance at his advances.
Concisely, the expository events of Act I present the conflict of the story and also foreshadow what could become of the relationship of Lady Windermere with Lord Darlington.