  Using chiral low energy theorems and elastic unitarity assumption, the
$\tau\to\pi K \nu $ decay is investigated. The vector and scalar $\pi K$ form
factors are calculated. It is found that the $\pi K$ spectrum is dominated by
the $K^*$ resonance. By measuring the forward-backward asymmetry, it is shown
that the S wave $\pi K$ phase shift can be determined near the $K^{*}$
resonance region. The calculated branching ratio and resonance parameters are
in good agreement with experiments.
