  We show that studying the single top quark production via the $W$--gluon
fusion process can provide unique information on: (i) the measurement of the
decay width $\width$; (ii) probing the symmetry breaking mechanism by measuring
the form factor of~\tbW; (iii) testing the Effective--$W$ Approximation prior
to supercolliders; (iv) testing CP violation by observing different production
rates of $\bar p p \ra t X $ and $\bar p p \ra \bar t X$; (v) testing CP
violation from the almost hundred percent longitudinally polarized top quark
decay; in addition to the measurement of the top quark mass. In contrast to the
recent claim by R. K. Ellis and S. Parke, we show that this process is
extremely useful at the Tevatron.
