  Double white dwarf (DWD) binaries are important for studies of
common-envelope (CE) evolution, Type Ia supernova progenitors and Galactic
sources of low-frequency gravitational waves (GWs). PTF J0533+0209 is a DWD
system with a short orbital period of Porb ~ 20 min and thus a so-called LISA
verification source. The formation of this system and other DWDs is still under
debate. In this paper, we discuss the possible formation scenarios of this
binary and argue that it is not likely to have formed through CE evolution.
Applying a new magnetic braking prescription, we use the MESA code to model the
formation of this system through stable mass transfer. We find a model which
can well reproduce the observed WD masses and orbital period but not the
effective temperature and hydrogen abundance of the low-mass He WD component.
We discuss the possibility of using H flashes to mitigate this discrepancy.
Finally, we discuss the future evolution of this system into a AM CVn binary
such as those that will be detected by space-borne GW observatories like LISA,
TianQin and Taiji.
