How many times does "Hui" appear in the text? What are their respective roles?

Four "Hui" appeared in the article four times: the first "Hui" appeared at the beginning of the article. The "Hui" here is just a straightforward introduction to the article. The author intends to pave the way and render the sad atmosphere. The second "back" appears in the scene where my father buys oranges. Firstly, the author uses the method of portrait description to outline the general appearance of his father, which echoes the bleak family situation mentioned above. Then the author describes his father's old age and difficult steps with delicate action descriptions, which is touching. The figure I saw for the third time appeared when my father and son said goodbye, mixed with the crowds coming and going. Saying goodbye at this time deepened the reader's impression of the back. The fourth time I saw my back, I appeared in the scene of reading my father's letter. My father was unhappy in his later years, and a few words in his letter were very sad. At the same time, the father's concern for his son remains the same, which makes the author "see the fat back of blue cotton robe and black cloth jacket in crystal tears", interweaving his ardent memory of his father with his own feelings.