Explain how Iago manipulates Roderigo in Act 2: Scene 1.

In Act 2, scene 1, Iago's comment that Othello and Desdemona "are well tuned" is a metaphor of harmonic music in which he uses to indicate the current harmony of Othello's marriage. However the comment is then followed by Iago's vow "to set down the pegs", which show his intentions to disrupt the harmony between them.
When Iago and Roderigo are left alone together, Iago sees this as an opportunity to manipulate Roderigo by telling him that "Desdemona is directly in love with him" (him being Cassio), because she must necessarily tire of Othello. However, although Roderigo finds it impossibl…