"Don't fail to do this, since you'll—" (IV.i.218)
I'm happy about that... Why, sweet Othello— (IV.i.227) (IV.i.231)
Maybe the letter upset him. I think they want him to go home and appoint Cassio governor in his place. (IV.i.224-226)
This Iago is extremely honest and good, and he knows a lot about human behavior. If it turns out that she really is running around on me, I'll send her away, even though it'll break my heart. Maybe because I'm black, and I don't have nice manners like courtiers do, or because I'm getting old. . . (III.iii.262-270)
I accept your devotion with my deepest love. I'll put you to the test right away. Within the next three days, I want to hear you tell me that Cassio's dead. (III.iii.476-480)
My friend Cassio is dead. It's done, because you request it. But let her live. (III.iii.481-482)
This scene, where Othello receives a letter from Lodovico saying he will be replaced by Cassio for the governor position, impacts the drama because it demonstrates Othello's desire of maintaining his reputation and his hate for Cassio. Othello is also shown to be violent with Desdemona, which indicates his change of behavior. This problem is an external conflict (person vs person), as the Venetian government is ordering Othello to go home and Cassio to take his place as governor, The letter caused Othello to change and be violent to others, for example, his wife. Othello, as the general, for now, wants to both go against the orders and leave Cyprus; however, he must decide what he wants to do with his future.
Othello is most likely thinking of killing Cassio to maintain his position as the governor of Cyprus. Desdemona is wondering why Othello is angry at her and why he slapped her; she does not understand the reason. Lodovico, Desdemona's cousin, may also be thinking Othello is very different from what is told about him.
This scene demonstrates an internal conflict (person vs self) where Othello is doubting himself to be the man for Desdemona and whether they should actually be married despite their races. It impacts the drama by giving a reason for Othello to leave Desdemona and propelling Iago's scheme. A reason Othello thinks his wife if cheating on him is when Iago explains to Othello that the missing handkerchief Desdemona was unable to give to her husband was seen to be in Cassio's possession instead, which caused Othello to hate him. By doubting how suitable a husband he is, Othello may be leaving his wife sooner than expected. In addition, Iago's scheme includes making Othello leave Desdemona and trust the villain more instead of her.
Othello is wondering whether he should be the right husband for Desdemona, as he notices the differences between himself and other courtiers who would have been better for her. He notices the contrasting race colors, the manners needed to be a man, and the age difference between him and Desdemona. Emilia and Desdemona enter the room after Othello finishes his soliloquy.
Othello is changing; he hates Cassio now because of the logical reasonings Iago gave to him. Iago is proud of his own actions, as his plan is going quite smoothly.
This scene, where Othello is "testing" Iago for the Lieutenant position by commanding him to kill Cassio within the next three days, demonstrates an external conflict (person vs person). It will propel the drama by propelling Iago's plan, and perhaps the loss of a character would affect the story in the future. Othello orders Iago to kill Cassio, which would be a win-win situation for them both; Othello would get to keep Desdemona to himself, and Iago would be Lieutenant without Cassio in the way to get back at him. However, in Act IV, Iago orders Roderigo to get his hands dirty instead of his own.