The poem has a very informal tone from the start, when the speaker insists that parents will "fuck you up." In contrast with the antiquated, rather lofty title ("This Be The Verse"), the informality and bluntness of the first line is striking, perhaps catching readers off-guard and encouraging them to connect with what the speaker says on a more immediate, personal level. After all, the speaker is concerned with what the poem presents as a universal problem—namely, that parents always end up causing their kids some kind of emotional harm.
If the poem's opening line seems harsh, it's worth noting that the speaker softens this initial statement by saying, in the second line, that many parents "may not mean to" cause their children harm. In doing so, the speaker makes it clear that this poem is not supposed to be read as an attack on bad parents. Rather, the poem simply sets forth the idea that all parents unavoidably complicate their children's lives, regardless of their intentions. In this way, the speaker highlights the cyclical nature of this kind of unhappiness, ultimately suggesting that it's difficult, if not impossible, to escape childhood without having experienced some form of emotional damage inflicted by one's parents.
This idea sounds quite pessimistic, and that's because it is pessimistic. At the same time, though, it might be helpful to bear in mind that the speaker's use of profanity in the opening line undercuts the seriousness of this otherwise grand assertion. After all, most people would probably give a little knowing chuckle after hearing the phrase, "They fuck you up, your mum and dad." Accordingly, the poem takes on a certain dry humor, even as the speaker makes such bleak claims. In this tongue-in-cheek manner, the speaker uses anaphora to repeat the word "they" at the beginning of the first two lines. In doing so, the speaker highlights the human tendency to constantly blame others for everything that has gone wrong in one's life. Indeed, parents are especially vulnerable to this kind of criticism, since so many people feel the same way the speaker does, believing that their parents have "fucked" them up.
Both lines 1 and 2 feature caesuras, which establish a distinct rhythm that is self-contained and terse. This is further accentuated by the fact that both lines are end-stopped, adding prominent pauses that, when combined with the meter of the poem, create a very musical effect.
To that end, these first two lines establish the poem's meter, since they are both in iambic tetrameter. This means that each line consists of four iambs, which are metrical feet consisting of an unstressed syllable followed by a stressed syllable (da-DUM). The lines scan like this:
They fuck | you up, || your mum | and dad.
They may | not mean | to, || but | they do.
The iambic rhythm in these lines gives the words a bouncy feel, one that is also created by the assonant sounds that pattern themselves throughout. For instance, the /uh/ sound repeats three times in the first line, appearing in the words "fuck," "up," and "mum." Similarly, the long /a/ sound repeats in the second line, appearing in the word "they" (which occurs twice) and the word "may."
All in all, this creates a satisfying sound that, when combined with the meter and pacing of the poem (in addition to the internal rhyme between "to" and "do" in the second line) ultimately gives the poem a sing-song quality. In this regard, the beginning of the poem sounds surprisingly cheerful despite the speaker's pessimistic outlook, thereby inviting readers to laugh at the sad notion that all parents mess up their children.