This refers to the time when the writer was a child still living under his father's care. AT: "When I was still a boy learning from my father" (See: [[rc://en/ta/man/translate/figs-explicit]])
Here the word "tender" refers to a young age at which the child is still weak. It forms a hendiadys with the word "only." AT: "the tender only child" (See: [[rc://en/ta/man/translate/figs-hendiadys]])
Here the word "heart" represents the person's mind. The writer speaks of remembering words as if the heart were holding on tightly to them. AT: "Always remember what I am teaching you" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-metaphor]])