  Gravitational-wave (GW) data can be used to test general relativity in the
highly nonlinear and strong field regime. Modified gravity theories such as
Einstein-dilation-Gauss-Bonnet and dynamical Chern-Simons can be tested with
the additional GW signals detected in the first half of the third observing run
of Advanced LIGO/Virgo. Specifically, we analyze gravitational-wave data of
GW190412 and GW190814 to place constraints on the parameters of these two
theories. Our results indicate that dynamical Chern-Simons gravity remains
unconstrained. For Einstein-dilation-Gauss-Bonnet gravity, we find
$\sqrt{\alpha_{\rm EdGB}}\lesssim 0.40\,\rm km$ when considering GW190814 data,
assuming it is a black hole binary. Such a constraint is improved by a factor
of approximately $10$ in comparison to that set by the first Gravitational-Wave
Transient Catalog events.
