"Er and liang"

In Chinese, there are two words for two.

The digit 2 is 二. This is used generally in numbers, when counting to ten and so on. When specifying quantities, however, 两 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 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 must be used.