Would you take a look at that? How could anyone ignore that hole? In fairness, Tom didn't ignore it. This is his favorite sweater, which he's had for decades. But because he didn't know how to mend it, he asked me to try. I looked at it with horror, and then ignored it, for a long time. A little too long, by the size of the hole.
That hole had clearly grown way beyond help from nonna's magic darning egg, and my first attempt at darning it with needle and a brown yarn I had on hand, by simply weaving in and through what was left of the sweater, resulted in a mess.
So I trimmed away the darning and cut the loose ends, making the hole neater. Then I started again. (Yes, it's the same sweater in all photos, but because I hardly ever have the time to start and finish a project in one go, the photos were taken on different days in different lighting conditions)
And I think in the end I got a good darning system for large holes. First I made long horizontal stitches across, anchoring them on the solid borders of the hole, about two per knit stitch. Then I turned my work sideways and stem-stitched, working first left to right, keeping the working yarn below the needle, and then back the other direction right to left, keeping the working yarn above the needle. I filled the hole with stem stitching that looks pretty much like knit stitching (for a specific tutorial on the stem stitch, check out Mary Corbert's video).
Not perfect, but I think nonna would approve of my attempt.