generic reference時的articles

From Wikipedia:

In English, nouns must in most cases be preceded by an article that specifies the presence or absence of definiteness of the noun. The definite article is the in all cases other than generic references, which use the zero article (i.e., the absence of an article), while indefiniteness is expressed with a or an for singular nouns or the zero article for plural or non-count nouns.

Generic reference in English is usually indicated by the absence of the definite or indefinite article; mass nouns are expressed in the singular, count nouns in the plural, e.g.:

Patience is a virtue (mass noun)
Dogs are animals (count noun)

Occasionally generic reference is expressed by the definite or indefinite article with count nouns, e.g.:

The dog is a domestic animal
A dog has four legs 

其實四種article都可用在generic reference上。不可數名詞，用zero article，可數名詞，用plural加-s，或加a/an，或加the. 

問題是，

a dog 和the dog有分別嗎？

可參考以下：http://www.englishforums.com/English/GenericReferenceArticle/mvwrg/post.htm

主要權威來源是：

"We can also generalise by talking about one example of a class, using a/an (meaning "any") with a singular countable noun.
- A baby deer can stand as soon as it's born.
- A child needs plenty of love.
Note that we cannot use a/an in this way when we are generalising about all of the members of a group together.
- The tiger is in danger of becoming extinct. (NOT 
A tiger is in danger of becoming extinct. The sentence is about the whole tiger family, not about individuals.)"

M. Swan. Practical English Usage. 3rd Edition. p. 60