watakushi-wa [[sensei-ga o-kaeri-ni-nari-mashi-ta koto]-ni musuko-ga kizuka-zu-ni i-ta koto]-o zan’nen-ni omotte-ori-mas-u .
I-TOP [[teacher-NOM HON-return-DAT-become-UH-PAST fact]-DAT my.son-NOM notice–NEG-DAT exist-PAST fact]-ACC regretful-DAT think-PROG-UH-PRES
I regret (polite) that my son didn’t realize (plain) that the professor left (polite).