"er" is the number 2 "liang" is generally used as the counter for two (quantity of) For example: "liang ping pijiu" (two bottles of beer); NOT "er ping pijiu".

Generally,these two words both mean TWO,and used as a number. ER(二) is more literary,and means "different" in some situations...ex.三心二意 ，并无二致 LIANG(两) is spoken,& always used with measure words......ex.两个人，两件衣服

二er (1)Number one plus one income: one plus one equals twenty。数目，一加一所得：一加一等于二。 (2)Two different: no bargain, the heart is faithful.两样，不同：不二价，心无二用，忠贞不二。 (3)Second, the poor,of a sort: second-class, second-hand, second-rate level.第二的，较差的：二等，二手货，二流水平。 两liang (1)Number one plus one and, often used in pairs, two things: two flowers, into two halves.数目，一加一的和，常用于成双的事物：两个人，两朵花，分成两半。 （2）Said the number of uncertainty, roughly equivalent to a few: look in the eyes, a few days, he have real skill.表示不定的数目，大致相当于几：看两眼，过两天，他真有两下子。 （3）Both sides: in a dilemma, satisfy both parties.双方：两难，两全其美。 （4）China's old system of unit of weight: 10 liang equal 1 catty, a combined 50 grams.市制重量单位：10两等于一斤，一两合50克。

二和两有他们的固定用法，多看多背 如：两个人，两本书，两朵花，两部电影 二斤八两，两斤八两，第二天，第二名，第二部电影，第二排 understand?