In Silas Marner, what hold does Dunstan have over Godfrey Cass? How does he use this hold?
In the novel, Dunstan holds Godfrey's secret marriage to Molly Farren (an opium addict) over his head. As the only one with full knowledge of Godfrey's unsavory connection to Molly, Dunstan is able to blackmail Godfrey into doing his bidding whenever he likes. If Godfrey so much as balks at anything Dunstan wants done, the latter just resorts to the time-tested threat of exposing the secret marriage to their father, Squire Cass.
For his part, Godfrey is intent upon preserving his reputation above all else; after all, he has his eyes on the beauteous Nancy Lammeter, and his goal is to eventually make her his wife. So, his disastrous secret marriage to Molly must never come to light.
Meanwhile, in one of his characteristic blackmail attempts, Dunstan threatens to expose Godfrey if he refuses to pay back a hundred pounds in rent money Dunstan has appropriated for his own. The truth is that Dunstan forced Godfrey to hand the rent money over to him. This puts Godfrey in a difficult position with their father, Squire Cass, who is a landlord. Because he is missing the hundred pounds in rent, Squire Cass has threatened to "distrain" or to seize the renter's property in order to exact payment.
Of course, the renter, Fowler, paid the rent a while back; however, it ended up in Dunstan's hands, and he isn't about to pay it back. Instead, he's charging Godfrey with doing the honors. To ensure that Godfrey does his bidding, Dunstan threatens to expose to Squire Cass Godfrey's secret marriage to Molly. In fact, it was Dunstan himself who goaded Godfrey into marrying Molly. With this disastrous match, Dunstan ensured that the ball would always be in his court when it came to dealings with Godfrey.
For his part, Godfrey enjoys his comfortable existence too much to risk exposure, and he eventually agrees to let Dunstan sell his prized horse, Wildfire, in order to cobble together the money for the rent.