  In this paper, we mainly investigate Lax structure and tau function for the
large BKP hierarchy, which is also known as Toda hierarchy of B type, or
Hirota--Ohta--coupled KP hierarchy, or Pfaff lattice. Firstly, the large BKP
hierarchy can be derived from fermionic BKP hierarchy by using a special
bosonization, which is presented in the form of bilinear equation. Then from
bilinear equation, the corresponding Lax equation is given, where in particular
the relation of flow generator with Lax operator is obtained. Also starting
from Lax equation, the corresponding bilinear equation and existence of tau
function are discussed. After that, large BKP hierarchy is viewed as
sub--hierarchy of modified Toda (mToda) hierarchy, also called two--component
first modified KP hierarchy. Finally by using two basic Miura transformations
from mToda to Toda, we understand two typical relations between large BKP tau
function $\tau_n(\mathbf{t})$ and Toda tau function $\tau_n^{\rm
Toda}(\mathbf{t},-\mathbf{t})$, that is, $\tau_n^{{\rm
Toda}}(\mathbf{t},-{\mathbf{t}})=\tau_n(\mathbf{t})\tau_{n-1}(\mathbf{t})$ and
$\tau_n^{{\rm Toda}}(\mathbf{t},-{\mathbf{t}})=\tau_n^2(\mathbf{t})$. Further
we find
$\big(\tau_n(\mathbf{t})\tau_{n-1}(\mathbf{t}),\tau_n^2(\mathbf{t})\big)$
satisfies bilinear equation of mToda hierarchy.
