Ordinal numbers
Ordinal numbers (序数词) — are numbers defining the position of something in a series, such as “first”, “second”, or “third”.
Ordinal numerals are formed by a combination of the suffix 第 
and a numeral.
and a numeral.Scheme
第
numeral
measure word
(noun)
Pay attention
We should put the measure word after both ordinal and cardinal numbers.
Examples
第一次
the first time
dì yí cì
第八辆汽车
the eighth car
dì bā liàng qìchē
Pay attention
When it’s an ordinal number, number two is only stated as 二 . You cannot say 第两.
. You cannot say 第两.With some words, ordinal numbers can be expressed without 第. For example, when we state the number of a floor or room in a building, the number of a unit/chapter in a textbook, a class year or course number in educational institutions, or describing members of a household; we don’t have to use 第.
Examples
二层
second floor
èr céng
423房间
room 423
sì'èrsān fángjiān
二哥
the second oldest brother
èrgē