1. Third person singular imperfect indicative of the pronominal verb 'entristecer-se', meaning to become or grow sad; to sadden oneself
She saddened herself with sad memories.
Ela entristecia-se com memórias tristes.
2. Used to describe a habitual or repeated action of becoming sad in the past
He would grow sad every time he thought of her.
Ele entristecia-se sempre que pensava nela.