Sentences can be divided into single sentences and complex sentences according to their internal structure. A single sentence is a linguistic unit that is acted as a phrase or word, has a specific intonation, and can independently express a certain meaning. A complex sentence is composed of two or more clauses that are related in meaning and do not constitute each other in terms of structure. There is usually a pause between each clause in a compound sentence, which is indicated by a comma, semicolon or colon in writing; there are isolated phonetic pauses before and after the compound sentence, which is indicated by a period, question mark, or exclamation mark in writing.