Key for exercise Perfect tenses 2

I've often thought that Stefan's success as a teacher is due to his eccentricity as much as his knowledge of the subject. From the first time he ever walked/stepped into a classroom, students have always loved him. They've probably never  met anybody who displays such an extraordinary mixture of enthusiasm and great personal warmth. It is also probably the first time they've met somebody who always wears a leather jacket and a scarf even at the height of summer.