thatclauses

That clauses, or that-clauses, are finite subordinate clauses headed by the complementizer that. They function as declarative content clauses, conveying information, beliefs, thoughts, or speech. They commonly serve as the complement of verbs (think, believe, say, know), adjectives (clear, certain, sure), or nouns (fact, possibility, belief). They can also appear as subjects or as post-nominal modifiers in noun phrases, as in That the earth is round is well known or the claim that he is honest.

In English, that is often optional after many verbs and adjectives, especially in informal contexts. For example, I think you are right and I think that you are right are both acceptable, though that may be retained in formal or careful prose: It is believed that the policy will succeed. After some verbs of reporting or perception, that is less likely to be omitted, such as She said that she would come.

Tense and modality in that-clauses can shift in reported speech or thought. For instance, He said that he would come shows backshifting from will to would. In non-reported contexts, the tense typically reflects the actual time of speaking, as in I know that he is here.

That-clauses are distinct from whether-clauses or if-clauses, which express questions or alternatives rather than declarative content. They are a core topic in studies of English syntax and complementizer systems.