Difference between "Er" and "liang" in Chinese

This is an introduction to the different uses of "2" in Chinese, namely 二 (èr) and 两 (liǎng).

Structure

In Chinese, there are two words for two.

The digit 2 is 二 (èr). This is used generally in numbers, when counting to ten and so on. When specifying quantities, however, 两 (liǎng) is used. This is when you want to say "two of" something.

A good rule is that if a measure word is used, use 两. 二 does not generally occur with measure words.

Also remember that two o'clock uses 两: 两点. It's two points on the clock, which is a quantity, so 两 is used.

Examples
Here are some common examples of 二 in action:

Sì jiǎn èr dé èr
四减二得二.

Four minus two equals two.

Here are some common examples of 两 in action: 

Dàfēng yìzhí guāle liǎngtiān-liǎngy.
大风一直刮了两天两夜。

The gale blew for two days and two nights.