  We proposed general scheme for construction of exact real periodical
solutions of mKP-1 equation via Zakharov-Manakov $\overline{\partial}$-dressing
method, derived convenient determinant formula for calculation of such
solutions and demonstrated how reality and boundary conditions for the field
$u(x,y,t)$ can be satisfied. We calculated the new classes of exact periodical
solutions of mKP-1 equation: 1. the class of nonsingular one-periodic solutions
or nonlinear plane monochromatic waves; 2. the class of two-periodic solutions
without imposition of any boundary condition; 3. the class of two-periodic
solutions with integrable boundary condition $u(x,y,t)\mid_{y=0}=0$. We
interpreted the third class of two-periodic solutions with integrable boundary
condition obtained by the use of special nonlinear superpositions of two simple
one-periodical waves as eigenmodes of oscillations of the field $u(x,y,t)$ in
semi-plane $y\geq 0$, the analogs of standing waves on the string with fixed
endpoints.
