  The comparison of form factors calculated from a single-particle current in
different relativistic quantum mechanic approaches evidences tremendous
discrepancies. The role of constraints coming from space-time translations is
considered here with this respect. It is known that invariance under these
translations implies the energy-momentum conservation relation that is usually
assumed to hold globally. Transformations of the current under these
translations, which lead to this result, also imply constraints that have been
ignored so far in relativistic quantum mechanic approaches. An implementation
of these constraints is discussed in the case of a model with two scalar
constituents. It amounts to incorporate selected two-body currents to all
orders in the interaction. Discrepancies for form factors in different
approaches can thus be removed, contributing to restore the equivalence of
different approaches. Results for the standard front-form approach ($q^+=0$)
are found to fulfill the constraints and are therefore unchanged. The relation
with results from a dispersion-relation approach is also made.
