  The ab initio theory of electronic excitations in atomically thin
[quasi-two-dimensional (Q2D)] crystals presents extra challenges in comparison
to both the bulk and purely 2D cases. We argue that the conventionally used
energy-loss function $-$Im $1/\epsilon({\bf q},\omega)$ (where $\epsilon$,
${\bf q}$, and $\omega$ are the dielectric function, the momentum, and the
energy transfer, respectively) is not, generally speaking, the suitable
quantity for the interpretation of the electron-energy loss spectroscopy (EELS)
in the Q2D case, and we construct different functions pertinent to the EELS
experiments on Q2D crystals. Secondly, we emphasize the importance and develop
a convenient procedure of the elimination of the spurious inter-layer
interaction inherent to the use of the 3D super-cell method for the calculation
of excitations in Q2D crystals. Thirdly, we resolve the existing controversy in
the interpretation of the so-called $\pi$ and $\pi+\sigma$ excitations in
monolayer graphene by demonstrating that both dispersive collective excitations
(plasmons) and non-dispersive single-particle (inter-band) transitions fall in
the same energy ranges, where they strongly influence each other.
