Question

why a magnet can attract a metal but why it can not repel the same metal ?

Answer 1

repulsion occurs wen othr body same charge

Answer 2

and when does attraction occurs ?

Answer 3

as iron chargeless its molecules rearange favouring attraction

Answer 4

attraction for unlike charge

Answer 5

This might be a bit advanced, but watch the beginning of this lecture and it is explained:
http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-11-magnetic-field-and-lorentz-force/