"The Other" begins with an anonymous speaker addressing a second person directly. This opening is fairly vague: the reader doesn't know who "she" or "you" is nor what their relationship to each other might be (nor what their relationship to the speaker is). There's also no explanation as to what this "she" had "too much" of, which suggests that the exact nature of this "too much" doesn't particularly matter; as the poem goes on, readers will get the sense that the "you" of the poem would never be happy so long as "she" had anything at all.
Listen to how the sibilance here adds a smooth hush to the opening line, perhaps suggesting the delicateness and stealth with which "you" took "some," smiling all the while:
She had too much so with a smile you took some.
That "smile" might suggest deception—that you pretended to be close to the woman's while stealing from her—or perhaps it simply shows that you were pleased with yourself for this theft.
Line 2 then begins with a preposition, which might make it sound like a continuation of line 1 ("you took some / Of everything she had"). Except, line 1 is firmly end-stopped with a period. Line 2 is thus really the start of a new clause; it means something more like, "You had none of the things she had."
The line would read more clearly had Hughes written, "Of everything she had, you had absolutely nothing"—but, of course, Hughes didn't write it this way! Instead, he's deliberately toying with syntax here to make the lines of the poem run together, blurring the poetic distance between this other woman and "you."
The enjambment across lines 2-3 ("you had / Absolutely nothing") adds to the effect, keeping the poem feeling slippery and disjointed. In starting line 3 with the phrase "Absolutely nothing," enjambment also calls attention to the vast chasm between these two people's experiences: whereas "she" had "everything," the person the speaker is addressing had nothing at all.
Notice the repetition in lines 1-3:
She had too much so with a smile you took some.
Of everything she had you had
Absolutely nothing, so you took some.
Diacope (the repetition of "She had" / "had") adds rhythm to the poem and also feels a little claustrophobic, as though your envy for what the woman ate away at the space between the two of you.
Meanwhile, epistrophe (the repetition of "you took some" at the ends of lines 1 and 3) suggests that this taking was hardly a one-time thing. The repetition implies a pattern; you took and took from this woman. Indeed, in line 4, the speaker says that "At first," you took "just a little." The qualifier "At first" implies that you would go on to take far more than "just a little."
In terms of form, notice how short this first stanza is in comparison to the subsequent two. This gives the poem's opening stanza an introductory feel; the fourth line feels like a springboard into the rest of the poem. The poet's use of free verse creates a conversational and intimate tone, which is fitting considering the speaker is addressing someone they seem to know an awful lot about.
Finally, a note on the poem's context: given that this poem appears in a collection inspired by Hughes's relationship with the poet Assia Wevill, many readers take "she" as Sylvia Plath, "you" as the Wevill, and the speaker as Hughes himself. In this reading, Wevill is envious of Plath's relationship with Hughes, her poetic skill and ambition, and so forth. While this context helps to ground the poem's ambiguous language, readers should note that it's not strictly necessary to interpret the characters this way (indeed, if one didn't know this context, it might sound like the speaker is talking to themselves).