Complementizers

Complementizer	Clause type headed	Finite / non-finite	Typical function of clause	Example
that	Declarative content clause	Finite	Complement / subject	She believes that he is honest.
Ø (zero)	Declarative content clause	Finite	Complement	She believes Ø he is honest.
whether	Interrogative content clause	Finite	Complement	I wonder whether she agreed.
if	Interrogative content clause	Finite	Complement	She asked if we were ready.
for	Infinitival content clause	Non-finite	Complement / extraposed subject	It is unusual for him to leave early.
that (exclamative use)	Exclamative content clause	Finite	Complement	It’s amazing that she won.
whether or not	Interrogative content clause	Finite	Complement	They discussed whether or not to proceed.
if only	Optative content clause	Finite	Complement	She wishes if only he were here.

Notes (CGEL-aligned):

• All listed items are complementizers when they head content clauses.

• for is restricted to infinitival clauses and licenses an overt subject.

• Multi-word forms (whether or not, if only) function as complex complementizers.

• Presence of a complementizer does not imply the clause is a complement; the clause’s function is determined by the selecting head.