Sentences are constructed according to grammatical rules and can be categorized into two main types: major and minor sentences. Major sentences are regularly formed and include phrases or clauses, while minor sentences are irregularly formed. Sentences can also be simple, containing one clause, or multiple, containing more than one clause. Multiple sentences can be compound, with clauses on the same level, or complex, with clauses at different levels. There are four levels that make up the grammatical hierarchy: sentences, clauses, phrases, and words.