1. 在並列連詞(and, but, for, nor, or, yet )前使用，用來連接句中的各分句。
The principal reason for this is that none of them possesses chlorophyll, andsince they cannot synthesize their own carbohydrates, they obtain their supplies either from the breakdown of dead organic matter or from other living organisms.
2. 用逗號來分隔一系列單詞、詞組和從句。
Furthermore the walls of fungal cells are not made of cellulose, as those of plants are, but of another complex sugarlike polymer called chitin, the material from which the hard outer skeletons of shrimps, spiders, and insects are made.
They are a major cause of structural damage to building timbers, a cause of disease in animals and humans, and one of the greatest causes of agricultural losses.
3. 逗號用來分隔與句子其他部分密切相連的簡短插入語或旁白。(較長的，更為突兀的或復雜的插入成分的則用破折號或圓括號。)
Chemical fertilization,for example, helps to produce better crops, but is harmful to the environment.
4. 在並列形容詞，即分別修飾同一個名詞的形容詞之間使用逗號，但也有的形容詞之間不加逗號的。
For men, heroism was usually described as bravery and the active, successfulovercoming of adversity.
5. 用逗號來分隔非限定性修飾語，即該修飾語對於句子的意義並非必不可少。非限定性修飾語與限定性修飾語的區別在於，它即使被省略，也不會改變句子的主要意思。
(1) 同位語
The canopy, the upper level of the trees in the rain forest, holds a plethora of climbing mammals of moderately large size, which may include monkeys, cats, civets, and porcupines.
(2) 從句
The canopy, the upper level of the trees in the rain forest, holds a plethora of climbing mammals of moderately large size, which may include monkeys, cats, civets, and porcupines.
7. 在以"月、日、年"為次序排列的日期間使用逗號。如果這種次序的日期出現在句子中間，則在"年"后也加上逗號。
Martin Luther King, Jr., was born on January 15, 1929, and died on April 4, 1968.
但在以"日、月、年"為次序排列的日期間，不使用逗號。
Martin Luther King, Jr., was born on 15 January 1929, and died on 4 April 1968.
在"月"和"年"之間，或"季節"和"年"之間，不使用逗號。
The events of July 1789 are as familiar to the French as those of July 1776 are to Americans.
發生在1789年7月的那些事件對於法國人來說，就像美國人對於發生在1776年7月的那些時間一樣熟悉。
8. 在表示地名時
Pandas come from China, Asia.
9. 用以將一導言的或轉變語氣的詞或短語(如therefore, however, by the way, for instance)與句中其余部分隔開。
On the other hand, fungi bring about the decomposition of dead organic matter, thus enriching the soil and returning carbon dioxide to the atmosphere.
10. 書寫對話時用於某某說等詞語前后。
"Come back soon," she said.
11. 用於短小的引語前。
Disraeli said, "Little things affect little minds."
轉自：http://edu.qq.com/a/20130725/008925.htm