In this passage Iago is thinking of ways that he can make Othello think that Desdemona has betrayed him by putting the napkin, which was a wedding gift for Desdemona from Othello, into the room of Cassio so Othello is mad at both of them.
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 Sulphur. I did say so:
Look, where he comes!
Paraphrase: I will leave this napkin in Cassio’s house, And let him find it. Little things like this to a jealous man can look like something. The Moor already has had his mind infected with my poisonous thoughts. Ideas can be like poison. At first you don’t taste the poison but once you start doing the act it burns into your mind like sulphur.
In this part of the book the Duke, Desdemona, Othello, and Brabantio are talking about how Desdemona was forced into the relationship with Othello but she says that she loves him so Brabantio who is Desdemona’s father goes to the Duke and they are all talking about it.
Let me speak like yourself, and lay a sentence,
Which, as a grise or step, may help these lovers
Into your favour.
When remedies are past, the griefs are ended
By seeing the worst, which late on hopes depended.
To mourn a mischief that is past and gone
Is the next way to draw new mischief on.
What cannot be preserved when fortune takes
Patience her injury a mockery makes.
The robb’d that smiles steals something from the thief;
He robs himself that spends a bootless grief.
Paraphrase: Let me speak so I can say something that will help you forgive these lovers. If you can’t change something, don’t cry about it. When you are passionate about something bad that has happened, you are setting yourself up for more bad things to happen. Someone that has been robbed can smile about his losses he is superior to the thief that robbed him, but if you cry you are just wasting your time.