  The aim of the present work is to study structure and gas kinematics in the
photodissociation regions (PDRs) around the compact HII regions S235A and
S235C. We observe the [CII], [13CII] and [OI] line emission, using
SOFIA/upGREAT and complement them by data of HCO+ and CO. We use the [13CII]
line to measure the optical depth of the [CII] emission, and find that the
[CII] line profiles are influenced by self-absorption, while the [13CII] line
remains unaffected by these effects. Hence, for dense PDRs, [13CII] emission is
a better tracer of gas kinematics. The optical depth of the [CII] line is up to
10 in S235A. We find an expanding motion of the [CII]-emitting layer of the
PDRs into the front molecular layer in both regions. Comparison of the gas and
dust columns shows that gas components visible neither in the [CII] nor in
low-J CO lines may contribute to the total column across S235A. We test whether
the observed properties of the PDRs match the predictions of spherical models
of expanding HII region + PDR + molecular cloud. Integrated intensities of the
[13CII], [CII] and [OI] lines are well-represented by the model, but the models
do not reproduce the double-peaked [CII] line profiles due to an insufficient
column density of C+. The model predicts that the [OI] line could be a more
reliable tracer of gas kinematics, but the foreground self-absorbing material
does not allow using it in the considered regions.
