Comparing 二 ("èr") and 两 ("liang")

“二”（èr）是基数词，表示数字“2”。“两”（liǎng）通常用于数量词“二”，主要用于量词或单位词之前。

"二" (èr) is a cardinal number, meaning "2". "两" (liǎng) is also a form of the numeral "two", primarily used before measure words or unit words.

“二”通常用在以下情况：1. 作为序数词，表示“第二”，如“二楼”、“第二名”。2. 在电话号码、门牌号、小数、分数、百分数中。3. 在两位数以上（十、百、千、万等位）的数字中，如“二十”、“二百”、“两千” （注意，在“千”、“万”等大单位前，通常用“两”来代替“二”，但用“二”也完全正确，只是“两”更常见）。4. 在某些固定词组中，如“说一不二”。5. 在报时间时，时、分、秒等数字。 "两"通常用在以下情况：1. 用于量词前面，表示“两个”，如“两个人”、“两本书”。2. 用于表示重量、长度、容积等单位词的前面，如“两公斤”、“两米”、“两升”。3. 在“千”、“万”等大数字单位前面，表示“两千”、“两万”，这是最常见的用法，但用“二千”、“二万”也对。4. 不能直接跟在“十”的后面表示“二十”，只能说“二十”，不能说“两十”。5. 在一些固定词组中，如“两回事”、“半斤八两”。

"二" is typically used in the following situations: 1. As an ordinal number, meaning "second," such as "二楼" (second floor) or "第二名" (second place). 2. In phone numbers, room numbers, decimals, fractions, and percentages. 3. In numbers two digits or more (tens, hundreds, thousands, ten thousands, etc.), such as "二十" (twenty), "二百" (two hundred), "两千" (two thousand – note that before large units like "千" (thousand) and "万" (ten thousand), "两" is more commonly used than "二", though "二" is also correct). 4. In certain fixed phrases, such as "说一不二" (to mean what one says). 5. When telling time, for hours, minutes, and seconds. "两" is typically used in the following situations: 1. Before a measure word, meaning "two of something," such as "两个人" (two people) or "两本书" (two books). 2. Before units of measurement for weight, length, volume, etc., such as "两公斤" (two kilograms), "两米" (two meters), "两升" (two liters). 3. Before large numeral units like "千" (thousand) and "万" (ten thousand), meaning "两千" (two thousand) or "两万" (twenty thousand), which is the most common usage, though "二千" and "二万" are also correct. 4. It cannot directly follow "十" (ten) to mean "twenty"; only "二十" is correct, not "两十". 5. In some fixed phrases, such as "两回事" (two different things) or "半斤八两" (six of one, half a dozen of the other).