Oh, beware, my lord, of jealousy! It is the green-eyed monster which doth mock the meat it feeds on. That cuckold lives in bliss who, certain of his fate, loves not his wronger, but, oh, what damned minutes tells he o'er who dotes, yet doubts- suspects, yet soundly loves!
I will in Cassio’s lodging lose this napkin And let him find it. Trifles light as air Are to the jealous confirmations strong As proofs of holy writ. This may do something. The Moor already changes with my poison. Dangerous conceits are in their natures poisons Which at the first are scarce found to distaste, But with a little act upon the blood Burn like the mines of sulfur.
I had been happy if the general camp, Pioneers and all, had tasted her sweet body, So I had nothing known. Oh, now forever Farewell the tranquil mind! Farewell content! Farewell the plumèd troops and the big wars That makes ambition virtue! Oh, farewell! Farewell the neighing steed and the shrill trump, The spirit-stirring drum, th' ear-piercing fife, The royal banner, and all quality, Pride, pomp, and circumstance of glorious war! And O you mortal engines, whose rude throats The immortal Jove’s dead clamors counterfeit, Farewell! Othello’s occupation’s gone.
Iago is telling Othello that jealousy is a terrible thing and that he shouldn't let the monster take over. But Othello is miserable and it is taking over.
Cassio is telling us the plan of what he is going to do with the handkerchief. And what it is going to do.
Othello is telling Iago that he rather her sleep with anybody other than Cassio. And that he rather not know about it.