You put it between a number and a noun. Basically it is called “counting word", and is indicated in dictionaries as "CL" or a classifier. You can't just put numbers before nouns in Chinese. You must insert a counting word. There are lots of these as well, so be patient and you'll come across them one by one.

its a little hard this phenomenon just exists in chinese. its a little like "a cup of water"----一杯水 个 is just like 杯 here another example, 两棵树----two trees. etc so you can see, in chinese, counting something is like this: number + a special character + what you are counting. in chinese, there are several special characters but which one shall we use? seriously, there is no a clear rule, the only way that you can control them is to practise more........:D I can tell you some normal rule about these special characters: 棵: for all kinds of trees one tree---一棵树 张: for something with a big and flat face a table---一张桌子 片: for something with a flat face but very thin a leaf---一片叶子 篇: for text a text---一篇文章 there are 把、条、只(for small animal chicken, duck, dog, sheep etc)、个、头(for big animal pig cow etc)、口 etc etc....... meet one, remember one ~~~~ you will understand all of them slowly...... good luck~~~~

Hahaha,actually，These are the language habits of the Chinese people，in the meaning， The "èr"and"liang" is almost the same.I simple tell you: 1.used "er" When you just reading some number. For example 2,20,2000 and so on of the all. But you can't read 20(er shi)into liang shi . all in all,unless 20,that others would can used"liang",but at the same time can used"er"if you like.^_^ 2.on the other hand.even if you can read "two apples"into "er ge ping guo",but more time,Chinese would read"liang ge ping guo" PS:"èr"is bad language in the Northeast China.^_^

when i want to kid others,I say "一头人" instead of "一个人"，haha,头 is used to count animals(一头牛，一头猪，) ,not human beings,when I use 一头人，it means the person i describe is not human but anmal (pig example) :P

这是个好问题.