Sentences in English require a Subject a noun and a Verb an action . Dog runs . Girl jumps . You can add more details to a sentence by adding adjectives , adverbs and prepositions ! The red boulder slowly rolls down the hill . But ! Verbs that are part of a adjective clause , such as that is covered in moss are considered to be adjectives . So I can ' t say The red boulder that is covered in moss . as a complete sentence . It ' s a subject , the noun that the sentence would be about , but the verb is there is just describing the boulder , not stating an action . I could say The red boulder that is covered in moss is large . and make a complete sentence . Why yes , I do teach English for a living ! <| sep |> Hey sorry man , someone gave me an offer I couldn ' t pass up .