Lycidas' death -- remember this is the poem that Milton didn't want to write, he was "forc'd" to write it -Lycidas' death allows the uncouth swain to grow up and to move on.
Yet once more, O ye Laurels, and once more Ye Myrtles brown, with Ivy never sere, I come to pluck your Berries harsh and crude, And with forc'd fingers rude, Shatter your leaves before the mellowing year.
It's this line, "And with forc'd fingers rude" - this is called a broken line or a half-line, and this broken line has been read, I think, rightly as Milton's indication to his reader that he's not even up to the task of writing a sonnet at this point.