Is the ordinal numbers of 21 "21st" or "21st"?

Both can be used, but the pronunciation is different. 21th = Twenty One 21st = Twenty First

ordinal numbers is a kind of numeral, which is mainly mentioned in English grammar and expressed as "which number" in Chinese. In addition, in the birthday, when describing the date of your birth, ordinal numbers is also used. For example: May-first (5 May 1st).

English, belonging to the west Germanic branch of Germanic family in Indo-European language family, evolved from the languages spoken by Germanic people who immigrated to the British Isles from Scandinavia such as Denmark, Germany, the Netherlands and surrounding areas, and spread to all parts of the world through British colonization of active communication.

Due to its contact with many national languages in history, its vocabulary has changed from monism to pluralism, its grammar has changed from "more inflections" to "less inflections", and its pronunciation has also changed regularly. According to the number of native speakers, English is the official language used by the most countries.

English is also the most widely used second language in the world and the official language of the European Union, with the largest number of international organizations and Commonwealth countries, and the third largest number of native speakers in the world, only less than the number of native speakers of Chinese and Spanish.