The ordinal form of 18 — not the counting number eighteen, but the word that shows place or order. The eighteenth person in a line is in position 18; the eighteenth of May is that date; her eighteenth birthday is the one after seventeen. You can use it as an adjective (the eighteenth century, the eighteenth floor) or as a noun (she is arriving on the eighteenth).

The ordinal form of 1818的序數形式——not the counting number eighteen不是用來數數的eighteen（十八），but the word that shows place or order而是表示順序、位置的字。The eighteenth person in a line排隊裡的the eighteenth person（第十八個人）is in position 18就是站在第18個位置；the eighteenth of Maythe eighteenth of May（五月十八日）is that date就是那個日期；her eighteenth birthdayher eighteenth birthday（她的十八歲生日）is the one after seventeen是十七歲之後的那個。You can use it as an adjective你可以把它當形容詞用 (the eighteenth century, the eighteenth floor)（the eighteenth century十八世紀、the eighteenth floor十八樓）or as a noun也可以當名詞用 (she is arriving on the eighteenth)（she is arriving on the eighteenth她十八號抵達）。

Though actually, the eighteenth century is the 1700s, which tidy maths would not lead you to guess, and learners mix that up all the time.

Though actually不過，the eighteenth century is the 1700sthe eighteenth century（十八世紀）其實是1700年代，which tidy maths would not lead you to guess這不是照著整齊的算法就猜得出來的，and learners mix that up all the time學習者常常會搞混。

It is a completely ordinary word, not formal and not slang. As a fraction, an eighteenth is one of eighteen equal parts, but the everyday uses are order, dates, and centuries.

It is a completely ordinary word這是一個完全普通的字，not formal and not slang不正式也不是俚語。As a fraction當分數用的時候，an eighteenth is one of eighteen equal partsan eighteenth是十八等分之一，but the everyday uses are order, dates, and centuries但日常用法主要是順序、日期和世紀。