Theory:

A slumber did my spirit seal—
I had no human fears.
She seemed a thing that could not feel
The touch of earthly years.
No motion has she now, no force—
She neither hears nor sees,
Rolled round in earth’s diurnal course
With rocks and stones and trees.
 
                      ~ William Wordsworth
Explanation:
 
The poet begins by making an implied reference to the fact that he is experiencing a considerable amount of sadness. Generally, when people get sad, it is their mind that is disturbed. But the poet goes to an extra step and says that his spirit is sealed with a tinge of melancholy. The spirit is the seat of emotions and character in a human being. The narrator's beloved is referred to be in slumber. Here, the slumber refers to the deep eternal sleep that is death. No one is aware of what happens after this phenomenon. But the beloved's missing has created a void in the narrator. He also refers to the fact that he had never expected Lucy's soul to depart. The death of the beloved has made the poet's heart to die as well metaphorically. The poet also states that he has lost all his feelings and ability to show emotions after the loss. When something terrible happens, people tend to get shocked beyond belief that drains their emotions. The slumber also refers to the fact that the poet has no feelings as though he is in eternal sleep. The poet has become numb and devoid of fear, as only when one yearns to live life does one exhibit fear. When there is nothing more to lose, one does not fear anything.
 
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Slumber refers to soul departing from the body
 
The beloved's soul has departed from earth and does not experience any human feelings or concepts. Once the soul has departed from the body, it is believed to get one with the cosmos and does not have fear, sentiments, or joy. Most importantly, it is not affected by the time schedules of the Earth. The day turning into night, or the seasonal changes do not affect it. The poet seems to be pointing out the fact that the soul is free and does not have to stick to any kind of commitments that is laid out by the Earth's inhabitants. Years may pass, and the soul is not affected by it. On the contrary, people on earth experience feelings and are affected by the time frames, making them miss their beloved every passing moment. The poet feels it unfair that his beloved has left him and does not have to go through the pangs of missing someone.
 
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The poet misses the beloved
 
People do not understand the blessing they are bestowed with as they can move their limbs and feel emotions. The poet says that his beloved lays motionless after life departed from her body. He says that her body does not have force now. The things that people miss about their loved ones is not their looks, or material possessions. It is rather the charisma and the personality, the force of life in them. It is hard to realise and accept that the person who was alive with so much life is no more and will not be exhibit the same vibe and energy. She cannot see or hear anything, as souls have no purpose in experiencing the sights and sounds of earth. They have higher things to look into, and these are trivial things. But the poet ends on a positive note, saying that people who die do not leave us completely, instead they become part of nature and hence stay with us forever. They are rolled into the daily happenings of the earth. They become fragments in the parts of nature such as rocks, soil and trees. The poet thus says that his beloved has spread around the cosmos, giving him company.
 
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The soul becomes part of nature
 
Words with difficult Meaning:
 
S.No
Words
Meaning
1
SlumberDeep sleep
2
SealFasten securely
3
SpiritThe non-physical part of a person which is the seat of emotions and character
4
Earthly yearsThe passing time and seasons in Earth
5
DiurnalPertaining to happening daily
6
ImpliedIndirect or hidden
7
MelancholyExtreme sadness
8
NumbHaving no feeling or emotions
9
DepartTo leave or exit
10
PangsThe pain that one experiences
11
FragmentsBroken pieces of a whole substance
12
CharismaEnthusiasm or charm
Reference:
National Council of Educational Research and Training (2006). Beehive. A Slumber did my Spirit Seal  - William Wordsworth (pp.136-137). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.