This poem is a definition poem; Dickinson wrote a number of poems defining words. A presentiment is a foreboding, a feeling that something is about to happen, usually something unpleasant or evil.
Dickinson uses the metaphor of a shadow to define a presentiment. Except for the first two words, the entire poem describes the shadow. It is described so concretely that an inattentive reader might think the poem was only a description of the shadow. The key to understanding this poem is to apply the details describing the shadow to a presentiment or premonition. How is a presentiment like a long shadow?
A long shadow appears before the person or thing that is casting it; so a presentiment gives warning or appears before the event. A long shadow indicates a setting sun; a presentiment may indicate death (the setting sun is a common symbol for death, and Dickinson uses it in this way in other poems). Grass is startled by the warning, just as an individual is startled by the foreboding. The warning is that darkness (something harmful or deadly?) is "about to pass" or happen.
She uses present tense to capture the sense of imminence (imminence: impending evil, some danger about to happen). Present tense indicates that the danger is not safely past or over.
Please just look at the first line of the poem; don't think about meaning. Question: What do you notice about its appearance?
Answer: It is considerably longer than the other three lines in the poem; Dickinson is duplicating the length of a shadow in the length of her line.
Dickinson also uses sounds to achieve her effects. What words alliterate in line 1? Does the alliteration emphasize important words/ideas, or has she made a mistake in alliterating these words? She uses a pattern of s sounds. Find all the words that contain s, regardless of its position in the word. Has she emphasized and connected important words and ideas by repeating a sound? The s sound can achieve a number of different effects: it may create a feeling of softness, suggest hush and silence, or remind us of the hissing of a snake. (You may have noticed that softness, hush, silence, hiss all contain s.) Do you think Dickinson used s to achieve a special effect, or is their presence merely coincidence?