Content clause

In grammar, a content clause is a subordinate clause that provides content implied or commented upon by its main clause. The term was coined by Danish linguist Otto Jespersen. They are also known as noun clauses.
English.
In English, 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.
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, as complements of predicative adjectives in clauses with linking verbs or in small clauses or as object complements. In this latter use, they are commonly postponed to the end of their main clause, with an expletive "it" standing in their original place as subject:
Here as before, a conjunction is almost always included, although it does not need to be "that":
Interrogative content clauses.
Interrogative content clauses, often called indirect questions, 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:
Such clauses correspond to direct questions, which are questions actually asked. The direct questions corresponding to the examples above are "What did you do? How did he manage it? Did I look that bad? Where are the files?" Notice how, in English (and in some other languages), different syntax is used in direct and indirect questions: direct questions normally use subject-verb inversion, while indirect questions do not. Reported questions (as in the last of the examples) are also subject to the tense and other changes that apply generally in indirect speech. For more information see interrogative mood and English grammar.
Indirect questions can serve as adjective and noun complements. Here, in English, they are generally introduced by a preposition, especially "of":
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: