The opening lines of the poem firmly establish it as an elegy, introducing the subject of death right away. Lines 1-3 depict the titular grandfather's epiphany (sudden realization) in the last "hour" before his death that "his heart had never spoken" openly, during all "eighty years" of his life.
Immediately noticeable is the fact that these lines are written in the third person, preventing the reader from having direct access to the grandfather's thoughts. There is therefore a high degree of ambiguity whenever the poem discusses the grandfather's inner world, especially since, as these lines establish, he is a highly reticent character who has never spoken intimately with his family, the speaker included. This epiphany that he comes to in his last hour could well be imagined by the speaker, as a way to project a sense of regret onto the grandfather that would justify the man's lifelong coldness.
This sense of distance is reinforced by the use of personification in the second line: it is the grandfather's "heart." rather than the man himself, who "had never spoken." The heart traditionally represents a person's deepest emotions, but the grandfather's character is such that he seems separated from these emotions, living 80 years without expressing them. Baxter also emphasizes the sheer length of time that this constitutes, with the phrase "eighty years of days"; this focuses attention on the huge number of days, which is 365 times the number of years—over 29,000.
Though the poem does not have a consistent meter throughout, it's worth noting that these lines almost conform to iambic trimeter, meaning there are three iambs—feet with a da DUM rhythm—per line:
He knew | in the hour | he died
That his heart | had nev- | er spoken
In eight- | y years | of days.
Although there is variation, this consists only in adding additional unstressed syllables (note that there are a few anapests—feet with a da da DUM rhythm—above). All three lines are consistent in each containing three stressed syllables. The iambic beat mimics the beating of the heart, which evokes both its metaphorical importance as the seat of deep emotions, as well as its vital place in keeping humans alive. The ceasing of the heartbeat causes death, which is this poem's starting point.