1. What is an auxiliary?
 
　　An auxiliary is an unstressed form word which performs the grammatical functions of structure, of tense or of mood, a special feature in Chinese language.  
 
1.1. Structural auxiliaries
 
　　“的” is used after an attributive to identify its attributiveness, e.g. 我的书 (my book), 中国的历史 (the history of China), or at the end of a nominal structure to form a noun phrase, namely “的-phrase”, e.g. 开车的 (man who drives), 红色的 (something red)
　　“地” is used after an adjective adverbial, e.g. 愉快地学习(study happily), 轻轻地落下来(fall gently down)
　　“得” is used after a verb or an adjective to introduce a completion, e.g. 跑得快(run quickly), 好得很(very good) 
 
1.2. Tense auxiliaries
 
　　“了” is used after a verb or an adjective to indicate the completion of an action or a change, e.g. 我收到了一封信 (I have received a letter), ·叶子黄了 (The leaves have turned yellow)
　　“着” is used after a verb to indicate continuity, e.g. 她唱着歌 (She is singing a song)
　　“过” is used after a verb to indicate past tense, e.g. 昨天我去过车站 (I was at the station yesterday) 
 
1.3. Mood auxiliaries
 
　　Mood auxiliaries can be used at the end of a sentence to show a mood or feeling:
 
　　“的” indicates certainty, as in 我不会忘记的 (I will never forget it)
　　“了” indicates a change or new circumstances, as in 天快亮了 (It will soon be dawn)
　　“吗” expresses interrogation, as in 你学过法语吗？(Did you study French?)
　　“吧” indicates consultation, suggestion, request, or command, as in 快点儿吧。(Hurry up, will you?) or expresses agreement or approval, as in 好吧，我答应你了 (OK, you have my word) or shows doubt or surmise, as in 你就是李先生吧？(You are Mr. Li, I suppose)
　　“呢” shows interrogation, preposition, etc. as in 我的书呢？(Where is my book?) or gives emphasis, as in 我才不会告诉你呢！(I am the last person to tell you that)
　　“啊” expresses admiration, as in 景色真美啊！(What a lovely view!) or used as a sign of confirmation, as in 这样做很好啊！(This way of doing things is wonderful indeed!) or expresses doubt, as in 他们真的来啊？(Are they really coming?) or indicates a short pause, as in 书啊、报纸啊、笔啊，摆了一桌子 (The desk is littered with books, newspapers, pens and whatnot) 
 

2. What is noteworthy about Chinese auxiliaries 
 
2.1. Multifunctional use
 
　　As a word class exclusively found in Chinese language, the auxiliary is featured by its multifunction in use. Plentiful and expressive auxiliaries make up for inadequate intonation in Chinese language. For example, a declarative sentence can be turned into an interrogative sentence by adding an auxiliary at the end of the sentence and without changing its intonation:
　　她是一个教师。 --- declarative sentence
　　她是一个教师吗？ --- interrogative sentence
　　Auxiliaries are widely used in daily life. Their complicated usage is really a headache to foreign leaners because these words sometimes can be sensed, but not explained in words. A foreign learner might be confused by such sentences with auxiliaries as:
　　这顿饭他吃了半个小时。 --- It sounds he has ended the meal
　　这顿饭他吃了半个小时了。 --- It sounds he will continue eating
　　Why the double use of auxiliary "了", which indicates the completion of an action usually, in a sentence results in the meaning of uncompletion?
　　As language is shaped by common practice, in order to have a good command of Chinese auxiliaries, the best way for you is practice, practice, and practice in an environment where Chinese is spoken.  
 
2.2. The difference of a structural auxiliary from a mood auxiliary 
 
　　The character "的" may function as a structural auxiliary (used after an attribute) or as a mood auxiliary (used at the end of a sentence); besides, the "了" may function as a tense auxiliary (used after a verb or an adjective) or as a mood auxiliary (used at the end of a sentence). You shouldn't mix them up if you happen on them at the end of a sentence. General speaking, a mood auxiliary indicates only a mood and can be omitted without changing the principal meaning of a sentence (e.g. we can say "我不会忘记的" or "我不会忘记", both are the same meaning); while a structural auxiliary is essential in sentence and can't be neglected anyway (e.g. we can't say "开车" instead of "开车的").