  We study the diffusion of tagged hard core interacting particles under the
influence of an external force field. Using the Jepsen line we map this many
particle problem onto a single particle one. We obtain general equations for
the distribution and the mean square displacement $<(x_T)^2>$ of the tagged
center particle valid for rather general external force fields and initial
conditions. A wide range of physical behaviors emerge which are very different
than the classical single file sub-diffusion $<(x_T)^2 > \sim t^{1/2}$ found
for uniformly distributed particles in an infinite space and in the absence of
force fields. For symmetric initial conditions and potential fields we find
$<(x_T)^2 > = {{\cal R} (1 - {\cal R})\over 2 N {\it r} ^2} $ where $2 N$ is
the (large) number of particles in the system, ${\cal R}$ is a single particle
reflection coefficient obtained from the single particle Green function and
initial conditions, and $r$ its derivative. We show that this equation is
related to the mathematical theory of order statistics and it can be used to
find $<(x_T)^2>$ even when the motion between collision events is not Brownian
(e.g. it might be ballistic, or anomalous diffusion). As an example we derive
the Percus relation for non Gaussian diffusion.
