Indicative sentential subjects obligatorily have complementizers while infinitival sentential subjects may or may not have a complementizer. Also if is possible as the complementizer of an object clause but not as the complementizer of a sentential subject.
If the sentential subject is finite and a complementizer does
not adjoin in, the assign-comp
feature of the
S0 node of the embedding clause and the root node of the
embedded clause will fail to unify. If a complementizer adjoins in,
there will be no feature-mismatch because the root of the
complementizer tree is not specified for the
assign-comp
feature.
The
comp
feature nil is split into two
assign-comp
features ind_nil and
inf_nil to capture the fact that there are certain configurations in
which it is acceptable for an infinitival clause to lack a complementizer
but not acceptable for an indicative clause to lack a complementizer.