This passage alludes to the Bible, Matthew 10.20: "Are not two sparrows sold for a farthing, and one of them shall not fall on the ground without your Father?" Meaning God has a plan for the least of us that we cannot escape.
"If danger be now, why then it is not to come. There's a predestinate providence in the fall of a sparrow."
As always, Q1 is short by comparison to F and Q2. But this, to me, is a particularly good example of where the brevity of Q1 enhances the impact because it is more readily understood in performance, especially by those unfamiliar with the play. If Shakespeare is meant to be performed not read, as Tim's old high school English teacher said, then the above is more accessible and impacting in performance than:
"Not a whit, we defy augury: there's a special
providence in the fall of a sparrow. If it be now,
'tis not to come; if it be not to come, it will be
now; if it be not now, yet it will come: the
readiness is all: since no man has aught of what he
leaves, what is't to leave betimes?"
Long or short, it is sweet text that hurts the heart in much the same way that Hamlet himself describes when he learns of the duel: "Believe me, Horatio, my heart is on the sudden very sore all here about."
And still, he rejects ‘augury,’ any attempt to read the tea leaves in order to take steps accordingly and instead accepts his destiny.
He knows, from the beginning when the Ghost appears to him, that he must, eventually, confront the King -- "oh cursed spite that ever I was born to set it right" -- and that it will cost him his life. Now he realizes the time is upon him.
With this line, Hamlet's restless anxiety of “to be or not to be” gives way to a “let be” of God’s plan. (Or is it Satan's? Does it matter? One doesn't exist without the other). And in this final, heartbreaking giving up/giving in there resides finally, paradoxically, also the resolve to act.