Copper is a transition element. ${\mathrm{HNO}}_3$ is nitric acid. When the concentrated nitric acid reacts with the copper metal then it formed the copper nitrate and nitrogen dioxide gas. So, the nitric acid will decompose into the nitrogen dioxide gas and copper nitrate.

Reaction :-

$\mathrm{Cu} + \mathrm{Conc}. {\mathrm{HNO}}_3\to \mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2 + {\mathrm{NO}}_2$

Hence, the main products formed when copper metal is reacted with concentrated ${\mathrm{HNO}}_3$ are $\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2; {\mathrm{NO}}_2$.