  On the basis of the recently developed universal decline law of classical
novae, we propose prediction formulae for supersoft X-ray on and off times,
i.e., t_{X-on} = (10 \pm 1.8) t_3 days and t_{X-off} = (5.3 \pm 1.4)
(t_3)^{1.5} days for 8 < t_3 < 80 days. We have determined the absolute
magnitude of our free-free emission model light curves and derived maximum
magnitude vs. rate of decline (MMRD) relations. Our theoretical MMRD relations
are governed by two parameters, one is the white dwarf (WD) mass and the other
is the initial envelope mass at a nova outburst; this second parameter explains
the scatter of MMRD points of individual novae. Our theoretical MMRD relations
are also in good agreement with the well-known empirical formulae. We also show
another empirical relation of M_V(15) ~ -5.7 \pm 0.3 based on the absolute
magnitude of our model light curves, i.e., the absolute magnitude at 15 days
after optical maximum is almost common among various novae. We analyzed ten
nova light curves, in which a supersoft X-ray phase was detected, and estimated
their WD masses. The models best reproducing simultaneously the optical and
supersoft X-ray observations are ONeMg WDs with 1.28 \pm 0.04 M_\sun (V598
Pup), 1.23 \pm 0.05 M_\sun (V382 Vel), 1.15 \pm 0.06 M_\sun (V4743 Sgr), 1.13
\pm 0.06 M_\sun (V1281 Sco), 1.2 \pm 0.05 M_\sun (V597 Pup), 1.06 \pm 0.07
M_\sun (V1494 Aql), 1.04 \pm 0.07 M_\sun (V2467 Cyg), 1.07 \pm 0.07 M_\sun
(V5116 Sgr), 1.05 \pm 0.05 M_\sun (V574 Pup), and a CO WD with 0.93 \pm 0.08
M_\sun (V458 Vul). The newly proposed relationships are consistent with the
emergence or decay epoch of the supersoft X-ray phase of these ten novae.
Finally, we discuss the mechanism of shock-origin hard X-ray component in
relation to the emergence of companion star from the WD envelope.
