In grammar, a content clause is a subordinate clause that provides content implied by, or commented upon by, its main clause. The term was coined by Otto Jespersen. There are two main kinds of content clauses: declarative content clauses (or that-clauses), which correspond to declarative sentences, and interrogative content clauses, which correspond to interrogative sentences.
Declarative content clauses can have a number of different grammatical roles. They often serve as direct objects of verbs of reporting, cognition, perception, and so on. In this use, the conjunction that may head the clause, but is often omitted:
Similarly with certain verb-like adjectives:
They also often serve as complements of nouns — both nouns corresponding to the above verbs, and nouns like fact, idea, and so on. Here, that is almost always included:
Finally, they can serve as subjects, or as direct objects of verbs that link them to adjectives or other predicatives. In this use, they are commonly postponed to the end of their main clause, with an expletive it standing in their original place:
Here as before, a conjunction is almost always included, although it does not need to be that:
Interrogative content clauses can be used in many of the same ways as declarative ones; for example, they are often direct objects of verbs of cognition, reporting, and perception, but here they emphasize knowledge or lack of knowledge of one element of a fact:
Semantically, they can serve as adjective and noun complements, but unlike their declarative counterparts, they are generally introduced by a preposition, especially of:
And like declarative content clauses, they are often postponed to the end of their main clause, with an expletive it standing in their original place, when they serve as the subject of a verb, or as the direct object of a verb that links them to a predicative: