The opening lines of the poem make use of apostrophe, as the speaker urges an unidentified person to refrain from grieving the speaker's own death for longer than it takes for the funeral bell to stop ringing. Consequently, the first three lines of this sonnet establish that the speaker is unsentimental when it comes to the prospect of dying, preferring to focus on the poem's anonymous addressee (that is, the speaker's beloved) and how this person will cope with the loss. In turn, the speaker emerges as a selfless, thoughtful person.
At the same time, the speaker subtly acknowledges that the mourning process can be quite sad. This is made evident by the language the speaker uses in line 2, describing the funeral bell as "surly" and "sullen." The word "surly" is an adjective that refers to something that is unpleasant or ill-tempered, and the word "sullen" is generally used to describe something depressing and morose. Accordingly, it's clear that the speaker recognizes that death is usually accompanied by sadness—in fact, it is precisely for this reason that speaker has written this poem, ultimately hoping to relieve the sorrow the lover will surely feel in the aftermath of the speaker's passing.
This, however, doesn't change the speaker's apparently unemotional acceptance of the fact that life leads to death. Indeed, the matter-of-fact tone in these lines suggests that the speaker has made peace with the idea of leaving behind the world of the living, knowing that all souls will someday "fle[e]" from physical existence.
Each of these three lines is also characterized by alliteration and sibilance. To that end, the first line features a brief instance of alliteration with the repetition of the /m/ sound ("mourn for me"), whereas the second line repeats the /s/ sound ("surly sullen"), and the third line repeats the /w/ sound ("warning to the world"). On the whole, this creates a measured but musical sound that echoes the sound of the funeral bell tolling away—an effect that is further emphasized by consonance, as the /l/ sound works through the second and third lines:
Than you shall hear the surly sullen bell
Give warning to the world that I am fled
In keeping with this musical quality, the poem's first three lines are also written in perfect iambic pentameter, meaning that they each contain five metrical meet made up of an unstressed syllable followed by a stressed syllable (da-DUM). In combination with the alliteration, sibilance, and consonance, this unbroken rhythm creates a pleasing sound that is somewhat surprising, considering that the speaker is talking about death. And this, in turn, shows readers just how undaunted the speaker is by the prospect of dying.