watakushiwa [[senseiga okaerininarimashita koto]ni musukoga kizukazuni ita koto]o zan’nenni omotteorimasu .
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).