The structure of The Tenant of Wildfell Hall is unique in that half of the novel takes place in the early 1820s and half is set in the late 1820s. As the first volume of the novel is set in the late 1820s (in the letter Gilbert is writing to Jack), the section that comes next (Helen’s diary entries from the early 1820s) is technically a flashback. Helen’s diary entries are written in the present tense and are not interrupted by narration from Gilbert from the actual “present” of the novel: two of the classic characteristics of flashbacks.
In the following passage, Gilbert introduces the flashback, preparing Jack (as well as the readers) for the extreme temporal jump:
I have [Helen’s diary] now before me; and though you could not, of course, peruse it with half the interest that I did, I know you would not be satisfied with an abbreviation of its contents and you shall have the whole, save, perhaps, a few passages here and there of merely temporal interest to the writer, or such as would serve to encumber the story rather than elucidate it. It begins somewhat abruptly, thus – but we will reserve its commencement for another chapter.
One of the effects of this flashback is to pull readers into Helen’s story and understand more acutely—via the fact that everything is happening in the “present”—the type of terror she experienced in her abusive marriage with Arthur.