Several grammatical constructs can be used as noun substitutes, including pronouns, nominal clauses, infinitive phrases and gerundive phrases. The most common substitution replaces a specific noun with a general pronoun, such as he, she or it.
The sentence, "He likes to swim," uses the infinitive phrase "to swim," in place of an actual noun. The sentence "He likes swimming," uses the gerundive "swimming" in the same way. A nominal clause is a group of words that uses a conjugated verb to act as a noun. In the sentence, "The fact that you swam ten miles is astonishing," the phrase "the fact that you swam ten miles" is a nominal clause and serves as a noun substitute.