Why there is no the in the second case? For me both of this cases look identical.

His father gave him a bag full of nails. (...) On the first day he hammered in 37 nails, but getting the(1) nails into the fence was not easy. As the days went by, he was hammering in less nails, and within weeks he was able to refrain from getting angry and from hammering the(2) nails.