The poem's opening lines establish its two central characters: a man and his lover. Given the title of the poem and the fact that these lovers have been together "year after year," it's safe to assume that they are married.
The speaker's opening statement, "He never learned her, quite," suggests that he has never managed to fully comprehend his own wife. Right away, this challenges any preconceived notions readers might have about marriage or long-term romantic relationships, questioning the assumption that being romantically involved with somebody for a long time always leads to an easy or simple dynamic.
The speaker introduces a metaphor for the husband's conception of his wife that will ultimately run throughout the poem:
[...] Year after year
That territory, without seasons, shifted
under his eye. [...]
In this context, the word "territory" metaphorically presents the wife's personality as a geographical feature, one that "shift[s]" right before the speaker's eyes. The idea of the wife as a moving piece of land implies that her fluctuations affect the speaker's ability to navigate their relationship—after all, it's pretty hard to become familiar with a landscape if it's constantly changing.
The phrase "without seasons" also plays into the metaphorical connection the poem makes between the wife and the natural world. Whereas seasons are predictable because they occur at the same time each year, the wife "shift[s]" in ways that seem, at least to the husband, random. In this way, the poem's opening lines establish the husband's sense of helplessness when it comes to understanding his wife.
The speaker uses diacope in the phrase "year after year," the repetition implying that the husband's inability to comprehend his wife is nothing new. She is, it seems, constantly changing. Ironically enough, then, one of the only constants in this marriage is the fact that it is always undergoing a transformation of some sort—a paradox that highlights the idea that romantic relationships unfold in unexpected ways and that this is one of the only predictable things about love.
These lines also establish the poem's loose adherence to iambic pentameter, a meter in which each line is made up of five iambs (metrical feet consisting of an unstressed syllable followed by a stressed syllable; da-DUM). The first line, for example, can be broken down like this:
He nev- | er learned | her, quite. | Year af- | ter year
The fourth foot of this line can be read as a spondee, or a foot made up of two stressed syllables, or a trochee ("Year aft-"). Either way, when combined with the caesuras after "her" and "quite," this disrupts the iambic rhythm, squandering the sense of predictability that the meter might otherwise lend to the poem. This, in turn, is appropriate for a poem about an unpredictable, volatile romantic relationship.