  The role of Poincar\'e covariant space-time translations is investigated in
the case of a relativistic quantum mechanics approach to the pion charge form
factor. It is shown that the related constraints are generally inconsistent
with the assumption of a single-particle current, which is most often referred
to. The only exception is the front-form approach with $q^+=0$. How accounting
for the related constraints, as well as restoring the equivalence of different
RQM approaches in estimating form factors, is discussed. Some extensions of
this work and, in particular, the relationship with a dispersion-relation
approach, are presented. Conclusions relative to the underlying dynamics are
given.
