  We investigate star formation from subparsec to kpc scales with
magnetohydrodynamic (MHD) models of a cloud structure and a section of galactic
spiral arm. We aim to understand how magnetic fields affect star formation,
cloud formation and how feedback couples with magnetic fields on scales of
clouds and clumps. We find that magnetic fields overall suppress star formation
by $\sim$10% with a weak field (5 $\mu$G), and $\sim50$% with a stronger field
(50 $\mu$G). Cluster masses are reduced by about 40% with a strong field but
show little change with a weak field. We find that clouds tend to be aligned
parallel to the field with a weak field, and become perpendicularly aligned
with a stronger field, whereas on clump scales the alignment is more random.
The magnetic fields and densities of clouds and clumps in our models agree with
the Zeeman measurements of the Crutcher relation $B-\rho$ in the weaker field
models, whilst the strongest field models show a relation which is too flat
compared to the observations. In all our models, we find both subcritical and
supercritical clouds and clumps are present. We also find that if using a line
of sight (1D) measure of the magnetic field to determine the critical
parameter, the magnetic field, and thereby also criticality, can vary by a
factor of 3-4 depending on whether the direction the field is measured along
corresponds to the direction of the ordered component of the magnetic field.
