Catherine is confiding in Nelly that she has accepted Edgar's marriage proposal, yet she's conflicted because she also loves Heathcliff—whom she could never marry:
It would degrade me to marry Heathcliff, now; so he shall never know how I love him; and that, not because he’s handsome, Nelly, but because he’s more myself than I am.
Nelly notices that Heathcliff is in the room, but he runs away when he overhears Catherine saying it would "degrade" her to marry him. Heathcliff's running away is an instance of dramatic irony because readers know, through Nelly's narration of Catherine's words, that Catherine passionately loves Heathcliff, but Heathcliff leaves before he hears Catherine's full declaration—she goes so far as to say that she and Heathcliff are effectively the same person, something she will never claim about Edgar.
The same passionate nature that fuels his love for Catherine leads Heathcliff to hastily abandon her and Wuthering Heights. Heathcliff's departure dooms whatever hope there could have been for a romantic relationship with Catherine, as she then follows through with marrying Edgar Linton instead. Moreover, it's ironic that a half-overheard, misunderstood sentiment leads to decades of misery for the families of Wuthering Heights and Thrushcross Grange. Furious about the way Hindley has degraded him in Catherine's eyes through mistreatment, Heathcliff later goes on to abuse Hindley's son Hareton, marry and mistreat Isabella Linton, and kidnap Catherine's daughter Cathy in revenge. Thus, Heathcliff's fleeing turns out to be the novel's turning point—though right now, what readers know is bad enough: that Heathcliff doesn't realize how much Catherine loves him and might never know.
When Isabella believes that she has fallen in love with Heathcliff, Catherine and Nelly try to warn Isabella about Heathcliff's true nature, but she doesn't heed them. Catherine uses nature imagery to convey that Isabella would be completely at Heathcliff's mercy.
"Tell her what Heathcliff is—an unreclaimed creature, without refinement—without cultivation; an arid wilderness of furze and whinstone. I'd as soon put that little canary into the park on a winter's day as recommend you to bestow your heart on him! [...] [H]e'd crush you, like a sparrow's egg, Isabella, if he found you a troublesome charge."
In particular, Catherine uses imagery associated with tame birds (a pet canary that couldn't fend for itself outdoors) or defenseless eggs (sparrow eggs are among the tiniest of wild birds' eggs) to warn Isabella that Heathcliff will treat her cruelly, and there will be nothing she can do to protect herself from him. In the same chapter, Nelly, too, advises Isabella to forget about Heathcliff: "He's a bird of bad omen; no mate for you." "Bird of bad omen" is an idiom that means Heathcliff is bad news, auguring a bad future for Isabella. In her more colloquial tone, Nelly builds on Catherine's more picturesque imagery to make clear that Heathcliff isn't the type of "bird" to match the delicate Isabella.
There's additional nature imagery in this passage: Catherine also associates Heathcliff with the wild, uncultivated Yorkshire moors. "Furze" refers to an evergreen shrub (gorse) that's ubiquitous on the moors; whinstone refers to dark, massive rocks common in the North of England. By contrast, Isabella has always been sheltered at Thrushcross Grange and carefully cultivated as a gentleman's daughter. She and Heathcliff are a complete mismatch, and unlike Catherine (whose choice of words suggests she's drawn to the wild moors, like Heathcliff), Isabella can't stand up to his rough, "unreclaimed" nature.
Dramatic irony is also at work in this passage, as readers know that Heathcliff is as bad as Catherine and Nelly say, but must watch Isabella naively blunder into a doomed marriage in the chapters ahead.
Cathy makes fun of Hareton for trying to teach himself to read. Readers know that Hareton has been struggling to learn to read—and choosing to recite Cathy's favorite passages—in order to impress her and win her affection, but Cathy ironically attributes his efforts to mockery.
'[...] he has no right to appropriate what is mine, and make it ridiculous to me with his vile mistakes and mis-pronunciations! Those books, both prose and verse, were consecrated to me by other associations, and I hate to have them debased and profaned in his mouth! Besides, of all, he has selected my favourite pieces that I love the most to repeat, as if out of deliberate malice!’
This instance of dramatic irony highlights how unkind Cathy can be, especially in the miserable environment of Wuthering Heights, and it stirs readers' sympathy for Hareton. Stuck there against her will, Cathy assumes that Hareton's fumbling attempts at kindness could only be malicious. In doing so, she also selfishly disregards the idea that anyone besides her might desire something better in life.
Note the language of consecration and profanation that Cathy attributes to her books. The books are "consecrated" to her because of their association with her father and Thrushcross Grange, but Hareton's mere attempt to read from the books "profanes" them. This language underscores Cathy's contempt for Hareton as beneath her because of his lack of education. However, it also sets up a surprising twist in Cathy and Hareton's relationship later, when Cathy will soften enough that she's actually willing to teach Hareton to read.
