Where does Offred realize the Latin phrase carved in her room was learned? What does this cause her to realize about the after-hours time she is spending with the Commander?

Offred asks the Commander what the Latin phrase in her bedroom means, hoping that he won't catch on to where she saw it. Though puzzled at first, he finally explains that it is a joke: it means, essentially, "Don't let the bastards grind you down." He shows her one of his old Latin schoolbooks, full of jokes like this one. Now she understands why the woman wrote it. She had been here, in this room with the Commander, as well. Offred asks him what happened to the woman who preceded her, and he tells her that she hung herself on the light fixture because Serena Joy found out about her relationship with the Commander.