The set phrases include idioms and abbreviated expressions. They function as a word in sentence and should be used as a whole without change of their forms. 
 
1 Idioms 

　　The Chinese language has many idioms whose meaning is not clear from the meaning of its individual words and which must be learnt as a whole unit. A great number of them (especially those that originated from the ancient Chinese) take the form of four characters to show a beauty in rhetoric. e.g.
 
 
　　半斤八两 (six of one and half a dozen of the other)
　　千方百计 (by every possible means)
　　走马看花 (gain a superficial understanding through cursory observation)
　　水落石出 (the truth prevailed in the end)
　　火上加油 (to add fuel to the fire)
　　混水摸鱼 (to fish in troubled waters)
　　趁热打铁 (strike while the iron is hot)
　　隔墙有耳 (walls have ears)
　　吃老本 (live off one’s past gains or achievements)
　　走过场 (make a gesture to give the impression of doing sth.)
　　竹篮打水一场空 (drawing water in a bamboo basket)
　　失败者成功之母 (failure is the mother of success)
 
 
2. Abbreviations 
 
　　A Chinese abbreviation is the shortened form of a phrase. In general, it is formed by using the first or key characters (syllable) of every or key words in phrase, e.g.
 
Abbreviation Original phrase In English
科技 科学技术 science and technology
外贸 对外贸易 foreign trade
劳保 劳动保护 labor protection
拉美 拉丁美洲 Latin America
北大 北京大学 Peking University
中共 中国共产党 the Communist Party of China 
欧盟 欧洲联盟 EU
教科文组织 联合国教育、科学及文化组织 UNESCO 

Sometimes, abbreviations can be formed by introducing a numeral, e.g. 
 
Abbreviation Original phrase In English
两广 广东和广西 Guangdong and Guangxi (China’s regions)
三北 中国东北、华北和西北地区 Northeast, Central North and Northwest China
四会 会听、会说、会读、会写 be good at listening, speaking, reading and writing (referring to language learning)