As in all literary questions, our ideas of what Sophy might think and write must be guided by the text and may be guided by what we know of the author's aesthetic theory, specifically the author's aims and objectives in expressing themes and characterizations.
One important part of Hardy's aim and objective is to expose his belief that life is ordered by a combination of accident, coincidence and luck. Aside from the advantages of birth and wealth, Hardy didn't find the workings of an overarching Fate or Providence in the events and circumstances of life (though there may be said to be some inconsistencies in his presentation of this aesthetic). As a result, Hardy would characterize Sophy through (1) thoughts and writing that would reflect accidental occurrences, coincidences and the influence of luck, either good or bad.
Textual evidence for what Sophy in fact did think after Randolph made her swear is scant. There are four references that might enlighten us before "[s]ome four years" pass and Sophy's funeral procession leads to her final resting place past a lonely, miserably mournful but lovingly faithful Sam. One of these four references to her thoughts tell us that at first she was hopeful that completion of Randolph's education and ordination as a minister of God (ironic, that) would cause him to change his mind or simply forget or neglect Sophy's plight. (2) Her thoughts and what she might have written would reflect this hope.
[Sophy was] thinking he would soften as soon as he was ordained and in full swing of clerical work.
Another reference tells us that it was not to be the case that Randolph might soften (on the contrary, he hardened his heart more) and, as a result of her increasing confinement, Sophy became worseningly lame and increasingly despondent.
Her lameness became more confirmed as time went on, and she seldom or never left the house
she seemed to be pining her heart away.
Finally, in her increasing despair and lonely submission, she seems to have gone a little distracted since we are told that she mutters the same sad questions over and over to herself when no one is around to hear.
'Why mayn't I say to Sam that I'll marry him? Why mayn't I?' she would murmur plaintively to herself
(3) Sophy's thoughts and writing might very well express (a) what might have helped her lameness from worsening, (b) what she might have longed to do, and, eventually, (c) further aimless repetitions.
While eNotes does not do assignments for students, I can give you a brief suggestion of what she might have written if we put all these considerations together:
- It was only coincidence and an accident that led me to marry the vicar. Had Sam not been so insistent and had we not quarreled and had I not fallen, things would have been different. Randolph will soften his heart after his ordination and see that his poor mother cannot disgrace him if I'm tucked away in a quiet little village. Would that I could ride with Sam in the air again.