grammar - The confusion between "The" or "A" article

I was reading about articles in Wren & Martin and found the following explanation:

1)Use the definite article when a singular noun is meant to express a whole class.

The cow is a useful animal

[Or we can say, "cows" are useful animals]

2)Use the indefinite article in the sense of "any" to single out an individual as the representative of a class; as,

A cow is a useful animal.

Can you please explain me the difference between 1 and 2.

I also found a given below sentence in wren & martyn's exercise.

__ lion is ___ king of beasts

As per above given explanation of definite and indefinite article, I am not sure which one should be correct: "The lion" or "A lion", or both are correct.

Could you please help me understand the difference better.

Answer

1) Use definite article, when a singular noun is meant to express a whole class:

The cow is a useful animal for the mankind
The cow is an animal species useful for the homo sapiens
(The) cows are useful animals for farmers

Here you are talking about classes (the cow species, the mankind, the farmers): "the cows (all of them as a group) are useful for the human beings (again all of them)".

2) Use indefinite article, in the sense of any, to single out an individual as the representative of a class:

A cow is a useful animal for a farmer
A herd of cows is useful for a farmer

In the last case you are not talking about class of the cows or the farmers, but about a single, indefinite (unknown or irrelevant the actual identity) cow (or herd of cows) and farmer.