8. $90^{\circ}$ As shown in the figure, in rectangle $A B C D$, let $A C$ intersect $M N$ at $O$. Connect $A^{\prime} O$.

According to the problem, points $A$ and $C$ are symmetric with respect to line $M N$. Therefore, $A O \perp M N, O C \perp M N$. After folding, $M N \perp$ plane $A^{\prime} O C$, so $M N \perp A^{\prime} C$.
Thus, the angle formed by line $M N$ and line $A^{\prime} C$ is $90^{\circ}$.
8. In rectangle $A B C D$, $A D=a, A B=b, b>a$, fold the rectangle so that point $A$ coincides with
point $C$, obtaining the crease $M N, M, V$ are on $A B, C D$ respectively, then unfold it, making the half-plane $M N D^{\prime} A^{\prime}$ form a $18^{\circ}$ dihedral angle with the half-plane $M N C B$, then the angle formed by line $A^{\prime} C$ and line $M N$ is $\qquad$.