Lines 1-2 together introduce the speaker's anxiety about the unavoidable passage of time. The ticking of a clock reminds the speaker that time is constantly passing, as does seeing beautiful daytime turn into "hideous," or ugly, night. The mention of day and night also introduces the more specific idea that, just as time inevitably passes, beauty inevitably fades.
Lines 1-2 establish the poem's meter of iambic pentameter, meaning that each line has five iambs (poetic feet with an unstressed-stressed beat pattern):
When I do count the clock that tells the time,
This steady meter will hold throughout the poem. The first line, in particular, is noteworthy for being astonishingly regular, with no variations in its meter, and for using only single-syllable words. The line's regular rhythm helps to suggest the relentless ticking of the clock that it describes. The strong alliteration and consonance ("count the clock" and "tells the time") only reinforce this impression, suggesting the constant, unavoidable passage of time which is the source of the speaker's distress.
This regularity and use of single-syllable words also make the next line all the more startling. The word "hideous" after a string of monosyllables helps to emphasize the idea of nighttime as something truly terrifying and horrible—which is appropriate, as it is also the poem's first image of decay.
Line 2 also sets up the rest of the poem's use of juxtaposition to emphasize both the disappearance of beauty and the ugliness it gives way to. Here, "brave," a word that suggests brightness and beauty, contrasts with the "hideous" quality of night.