When do you use 'er' and when do you use 'liang' in Chinese language?

Answer 1

I agree with the second answer, here, I want to add several points, "二(èr)" and "两(liǎnɡ)" are both numeral words, but their usages are not the same.

1. In the end of multi-digit, "二(èr)" is only used. For example: "七十二个" can be pronounced "qīshí'èr ɡè" but not 七十两个(qīshí liǎnɡ ɡè).

2. In the middle of a multi-digit, "二(èr)" is only used. For example: "八百二十八" can be pronounced "bābǎi èrshíbā" but not "bābǎi liǎnɡ shí bā."

3.Both "二(èr)" and "两(liǎnɡ)" can be used at the beginning of a multi-digit, Such as ＂二百(èrbǎi)＂also can be pronounced as ＂两百(liǎnɡbǎi).＂

4.There are some differences when they are used with measure words, "两(liǎnɡ)" can be used with　个(ɡè ,),把(bǎ),块(kuài,),棵(, kē,),只(zhī),条(tiáo) and so on, but "二(èr)" can't be used with them, For example: ＂两个(liǎnɡ ɡè)＂＂两条(liǎnɡ tiáo).＂Both "二(èr)" and "两(liǎnɡ)" can be used with斤(jīn), 里(lǐ), 尺(chǐ) and吨(dūn), such as两斤(liǎngjīng) also can be pronounced as二斤(èrjīng).

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Answer 2

You use "er" 二 when stating a specific quantity of two items or people. You use "liang" 两 when stating a general quantity of two items or people. For example, you say "两个人" (liang ge ren) to mean "two people" and "这是二本书" (zhe shi er ben shu) to mean "these are two books."