couldn't understand why
Tom no podía entender por qué no le gustaba a Mary.
Tom couldn't understand why Mary didn't like him.
Tom no podía entender por qué Mary se reía de él.
Tom couldn't understand why Mary was laughing at him.
Tom no podía entender por qué nadie más parecía estar contento.
Tom couldn't understand why no one else seemed to be happy.
Tom no podía entender por qué Mary se tenía que ir.
Tom couldn't understand why Mary had to leave.