When introducing readers to Lydgate, Eliot uses a flashback, as introduced by the narrator in the following way:
For those who want to be acquainted with Lydgate it will be good to know what was that case of impetuous folly, for it may stand as an example of the fitful swerving of passion to which he was prone, together with the chivalrous kindness which helped to make him morally lovable. The story can be told without many words. It happened when he was studying in Paris.
In the flashback, Eliot tells the story of how Lydgate—while studying abroad in France—fell in love with an actress who intentionally stabbed her husband (a fellow actor) while performing in a play together. After Lydgate follows the actress to another part of France—assuming her to be innocent—she admits that she stabbed her husband on purpose. Shocked, Lydgate decides to leave her and to give up on seeking out romantic love in the future.
This flashback provides important context for Lydgate’s character—readers leave understanding that he has a pattern of having his idealism turn to disappointment (which foreshadows what will happen with both his romantic relationship with Rosamond and his professional work at the New Hospital).