>The giraffe is the tallest of all animals.
>The bicycle is an excellent means of transport
>A child needs a mother...
There are three ways of using generic countable nouns:
(1)The+singular noun
(2)A+singular noun
(3)(No article) plural noun
Each has its own special meaning and therefore functions differently in a generic sentence; at times, they are fully interchangeable; at times, they are not.
(1)The+singular noun (e.g. the giraffe)
The giraffe represents all the typical qualities, features, and characteristics of a class of animals known as giraffes. In other words, "the giraffe" encompasses (is a bag of) all the properties that something must have and that we deem necessary to call the something a giraffe.
For example, we know that all the animals known to us as giraffes are tall when mature, but baby giraffes are also relatively tall; this gives us the right to call all giraffe species tall.
The giraffe is tall.
We know that all the things that we call "giraffe" are mammals. That’s why we can say "the giraffe" is a mammal.
The giraffe a mammal.
We also know that the species of giraffe didn't died out. That's why we can say that the giraffe are not extinct.
The giraffe is not extinct.
All of this means that "the giraffe" = [being tall, and being an animal, and being a mammal, plus many other characteristics that you think are attributable to the class of species known as "the giraffe."]
Actually, "the giraffe" doesn't exist as an embodiment. It's just the name of the set of its features.
(1)A+singular noun (e.g. a giraffe)
"A giraffe" refers to the property or properties that are indispensable for us to name something a giraffe. The word "indispensable" is the key: the property must be a defining property. Consequently, when the generic "a giraffe" is used, the feature referred to must be definitional.
"A giraffe is a mammal" is fine.
"A giraffe is tall" is fine.
"A giraffe is not extinct" is not fine, because not being extinct isn't a defining feature of giraffehood.
I have to run now.