In an inflected language, words change form according to grammatical function - this is called inflection.
Contrast isolating languages, which present the same information with word order and helper words more often than highly inflected languages do. Often the unmodified word root is a valid word by itself. However, distinguishing helper words from prefixes or suffixes in some languages (such as Japanese) can bring difficulty.
Several Native American languages are perhaps the most highly inflected languages known. The Navajo language is famous for its use by the United States during World War II as a spoken code. Other highly inflected languages include Mohawk, Inuktitut and Nahuatl. These languages inflect words to such a degree that a single word is often translated as an entire sentence in most other languages. A Mohawk word often given as an example is Washakotya'tawitsherahetkvhta'se, which means "He made the thing that one puts on one's body ugly for her", i.e., "He ruined her dress." Such highly inflected languages are also called polysynthetic languages.
Other examples of highly inflected languages include Latin, Greek, Arabic, Czech and Russian. For instance, in Russian, which distinguishes between grammatical genders, the adjective stem odin, "one" can be inflected in the following ways:
On a continuum from highly inflected to highly isolating, most modern Indo-European languages languages lie toward the inflected end. For example, Spanish, French, German, and the Scandinavian languages all inflect nouns and adjectives according to grammatical gender. These modern tongues make considerably less use of inflection than the archaic languages (such as Latin) from which they evolved.
In contrast, the Chinese language lies near the isolating end of the continuum. Even grammatical functions like plurals and possessives are expressed using individual words.
Modern English is often cited as a language that does not use much inflection. This is true for common nouns: English nouns have no grammatical gender and case is represented by word order, although most nouns are inflected for number. But there are many exceptions among the pronouns: for example, who, whose, and whom are inflected nominative, genitive, and accusative forms of the same word. English has a great many irregular verbs (161), whose multiple forms must be memorized; these are generally more inflected than regular verbs. Consider, for example, the irregular verb write and the regular verb push: "she wrote" (past tense) and "she had written" (past perfect tense), versus "she pushed" (past tense) and "she had pushed" (past perfect tense). Modern English also has inflection of person for some verbs, e.g. "I am", "you are", "he/she/it is". These irregular constructs reflect a past in which older forms of English were much more highly inflected than modern English.
Western European languages generally tend to become less inflected over time. One source of pressure to drop inflection is the development of pidgins and creoless. Closely related languages tend to have many roots in common, but to use different inflection systems. When two cultures meet, it is discovered that communication is possible simply by speaking in word roots, and dropping the inflection. The result of this process is an artificial language for interchange, called a pidgin. Sometimes the two cultures continue to mix over a long period of time, and children begin to speak the pidgin natively. This forces the language to grow, in order to provide service as a complete native language. When this happens, linguists would say that the pidgin has become a creole.
The English language has in fact been creolized more than once in its history, for example. Old English was a Germanic language, related to the language of Norse invaders who conquered part of England around 1000 A.D.. A pidgin evolved to facilitate communication between the English and the Norse, which resulted in the loss of many forms of inflection used in Old English. The complete history of the English language presents some problems, but the consensus of linguists is that English has undergone this process more than once in history. Similar pressures and other factors lead some languages to shed inflection over time.
See Typological classification