You have correctly cited the general rules for the usage of the definite article “the,” the indefinite articles “a” and “an,” and the zero article (where no article is used). I’m afraid though that it’s so elusive to come up with a hard-and-fast rule on how to use them when making generalizations about people, animals, places, things, and abstract concepts. I’d rather that the choice of article be made on a case-to-case basis depending on point of view or temperament of the writer or speaker, the nature of the noun, and the semantics of the statement being made.
Consider the three statements you provided:
“
Whales belong to the order Cetacea.” (Plural, zero article)
“
A whale is shaped like a submarine.” (Singular, indefinite article “a”)
“
The whale has an average life span of 30 - 70 years.” (Singular, definite article “the”)
You’ll find that the three general statements above can be correctly rendered as well in this manner:
“
The whale belongs to the order Cetacea.” (Plural, definite article “the”)
“
Whales are shaped like a submarine.” (Plural, zero article)
“
Whales have an average life span of 30 - 70 years.” (Plural, zero article)
Or in this manner:
“
A whale belongs to the order Cetacea.” (Singular, indefinite article “a”)
“
The whale is shaped like a submarine.” (Singular, definite article “the”)
“
A whale has an average life span of 30 - 70 years.” (Singular, indefinite article “a”)
Based on the permutations of article usage for the statements above, I think you’ll agree that it’s not advisable or warranted to draw up and impose a general rule on how it should be done. The only useful guide I can think of when making a generalized statement about a noun is to decide ahead of time whether to use its singular or plural form.
While we are on the subject, I think it will be much more instructive and useful to quote verbatim here the definitions and usage rules provided by the
Merriam-Webster’s 11th Collegiate Dictionary for the articles “the” and “a,” as follows:
the
1 a — used as a function word to indicate that a following noun or noun equivalent is definite or has been previously specified by context or by circumstance <put the cat out> b — used as a function word to indicate that a following noun or noun equivalent is a unique or a particular member of its class <the President> <the Lord> c — used as a function word before nouns that designate natural phenomena or points of the compass <the night is cold> d — used as a function word before a noun denoting time to indicate reference to what is present or immediate or is under consideration <in the future> e — used as a function word before names of some parts of the body or of the clothing as an equivalent of a possessive adjective <how’s the arm today> f — used as a function word before the name of a branch of human endeavor or proficiency <the law> g — used as a function word in prepositional phrases to indicate that the noun in the phrase serves as a basis for computation <sold by the dozen> h — used as a function word before a proper name (as of a ship or a well-known building) <the Mayflower> i — used as a function word before a proper name to indicate the distinctive characteristics of a person or thing <the John Doe that we know wouldn’t lie> j — used as a function word before the plural form of a surname to indicate all the members of a family <the Johnsons> k — used as a function word before the plural form of a numeral that is a multiple of ten to denote a particular decade of a century or of a person’s life <life in the twenties> l — used as a function word before the name of a commodity or any familiar appurtenance of daily life to indicate reference to the individual thing, part, or supply thought of as at hand <talked on the telephone> m — used as a function word to designate one of a class as the best, most typical, best known, or most worth singling out <this is the life> <the pill> — sometimes used before a personal name to denote the most prominent bearer of that name
2 a (1) — used as a function word with a noun modified by an adjective or by an attributive noun to limit the application of the modified noun to that specified by the adjective or by the attributive noun <the right answer> <Peter the Great> (2) — used as a function word before an absolute adjective or an ordinal number <nothing but the best> <due on the first> b (1) — used as a function word before a noun to limit its application to that specified by a succeeding element in the sentence <the poet Wordsworth> <the days of our youth> <didn’t have the time to write> (2) — used as a function word after a person’s name to indicate a characteristic trait or notorious activity specified by the succeeding noun <Jack the Ripper>
3 a — used as a function word before a singular noun to indicate that the noun is to be understood generically <the dog is a domestic animal> b — used as a function word before a singular substantivized adjective to indicate an abstract idea <an essay on the sublime>
4 — used as a function word before a noun or a substantivized adjective to indicate reference to a group as a whole <the elite>
a
1 — used as a function word before singular nouns when the referent is unspecified <a man overboard> and before number collectives and some numbers <a dozen>
2 : the same <birds of a feather> <swords all of a length>
3 a — used as a function word before a singular noun followed by a restrictive modifier <a man who was here yesterday> b : ANY <a man who is sick can't work> c — used as a function word before a mass noun to denote a particular type or instance <a bronze made in ancient times> d — used as a function word before a proper noun representing an example or type <the attractions of a Boston or a Cleveland> e — used as a function word before a proper noun to indicate limited knowledge about the referent <a Mr. Smith called to inquire about the job> f — used as a function word before a proper noun to distinguish the condition of the referent from a usual, former, or hypothetical condition <a triumphant Ms. Jones greeted her supporters>
4 — used as a function word with nouns to form adverbial phrases of quantity, amount, or degree <felt a bit tired>
usage In speech and writing a is used before a consonant sound <a door> <a human>. Before a vowel sound an is usual <an icicle> <an honor> but especially in speech a is used occasionally, more often in some dialects than in others <a apple> <a hour> <a obligation>. Before a consonant sound represented by a vowel letter a is usual <a one> <a union> but an also occurs though less frequently now than formerly <an unique> <such an one>. Before unstressed or weakly stressed syllables with initial h both a and an are used in writing <a historic> <an historic>. In the King James Version of the Old Testament and occasionally in writing and speech an is used before h in a stressed syllable <an huntress> <an hundred> <children are an heritage of the Lord — Psalms 127:3(Authorized Version)>.
I think all English learners, whether native or nonnative speakers, will become more confident and fluent in their English by studying and internalizing these specific usages of “the,” “a,” and “an.”
