The poem's first long sentence sets the scene: it's a summer's day on a hilltop, and a mouse is "crouched," hoping to make a break for it and evade the claws of a predatory cat.
The first line creates atmosphere through imagery. The hilltop is "sheep-cropped," meaning its grasses have been nibbled short by sheep. Though this might seem like an insignificant detail, it subtly introduces the poem's main theme: the natural world is governed by ancient instinct, not by reason. The sheep eat the grass because it's their nature to do so, just as a lurking predator might snap up a tasty mouse without a second thought. And a world ruled by sheep-eat-grass, cat-eat-mouse instinct, the poem will go on to suggest, is a pretty terrifying one to live in!
The "hot sun" beating down already hints at that terror and anxiety. Thick /uh/ assonance in this line—"summit, under hot sun"—makes the moment feel heavy with tension.
The poem's main verb doesn't arrive until line 2: "The mouse crouched." This subtle delay creates a mood of suspense: the mouse is pausing to weigh up its options. The /ow/ assonance in "mouse crouched" links the little creature with its posture, suggesting its tension as it considers whether to scurry out into the open or not.
This humble little country mouse, then, is contemplating a risk which, in the end, "it dare[s] not take." That is, it can see an opportunity, but its instincts are telling it to hold back. An enjambed line here mirrors its anxiety:
[...] the chance
It dared not take.
That mid-sentence line break leaves the reader in suspense for a moment, waiting to see what the mouse will do. In the end, it seems, the risk is just too great: this mouse is frozen in terror.
Throughout the poem, flexible free verse reflects the action in its form. Without a regular meter, the poem can stop and start as abruptly as a mouse thinking about making a dash for it.