This paper investigates clausal complements of factive and non-factive predicates in English, with particular focus on the distribution of overt and null that complementizers. Most studies on this topic assume that both overt and null that clauses have the same underlying structure and predict that these clauses show (nearly) the same syntactic distribution, contrary to fact: while the complementizer that is freely dropped in non-factive clausal complements, it is required in factive clausal complements by many native speakers of English. To account for several differences between factive and non-factive clausal complements, including the distribution of the overt and null complementizers, we propose that overt that clauses and null that clauses have different underlying structures responsible for their different syntactic behavior. Adopting Rizzi's (1997) split CP (Complementizer Phrase) structure with two C heads, Force and Finiteness, we suggest that null that clauses are FinPs (Finiteness Phrases) under both factive and non-factive predicates, whereas overt that clauses have an extra functional layer above FinP, lexicalizing either the head Force under non-factive predicates or the light demonstrative head d under factive predicates. These three different underlying structures successfully account for different syntactic patterns found between overt and null that clauses in various contexts.

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