numeral, number word belonging to which word class

Question

According to available information, there are eight word classes (=part of speech) in English:

noun, adjective, adverb, verb, preposition, pronoun, interjection, conjunction

What is a word class of the following words: four, eight, ten?

In my native language a numeral is a word class. In English it is not. So I am thinking, into which of the eight ones mentioned do these three words belong.

Thank you.

Answer 1

Those are numerals. Numerals are a word class in English too.

In particular they are cardinal numerals. Ordinal numerals (first, second, third etc) are a related word class.

Numerals share some characteristics with determiners (words like "the" "my", "some") This word class is missing from your list.

So your list is incomplete.

A simple analysis of word classes might be:

Major classes: Noun, Verb, Adjective, Adverb.

Minor classes: Preposition, Pronoun, Determiner, Conjunction, Interjection.

But this is not the only system. Some split up the "adverb" category. Some systems have classes like "subordinator" and "relativisor".

Most of these classes have sub-classes, for example, "personal pronouns" are a sub-class of the pronouns. And numerals can be classed as a sub-class of determiners.

Answer 2

It is possible to categorize all number words according to the standard eight parts of speech. This isn't necessarily the best approach, as @James K has stated, since experts consider the "8 parts of speech" list to be incomplete. But, it's still a useful idea because it keeps things simple and encourages classifying words based on both their meaning and their use in context. So, here are the rules that I learned:

• Cardinal numbers (one, two, three etc.) can be either nouns or adjectives, depending on how they are used in the sentence.
  Noun example: "Two is the only even prime number." ("two" is the subject of the sentence)
  Adjective example: "There are three geese on the pond today." ("three" modifies "geese")
• Ordinal numbers are usually adjectives.
  Example as attributive adjective: "Alice won first place."
  Example as predicate adjective: "Carol was second."

It is also sometimes possible for a cardinal number to function as a pronoun: an example would be "Bob likes eggs. He had two for breakfast." (Here, "two" functions as the direct object in the sentence, and its antecedent is "eggs.")

Ordinal numbers can also sometimes function as nouns (see the second linked reference), or as adverbs (although it's more common to add the -ly ending than to use them as "flat adverbs"; the reference cites both usages).

Here are references for the noun/adjective/adverb definitions:
https://www.dictionary.com/browse/three
https://www.dictionary.com/browse/fourth

And here is one that prefers them to be classified as "determiners":
https://www.scribbr.com/frequently-asked-questions/are-numbers-adjectives

And an interesting discussion that mentions both systems: https://en.wikipedia.org/wiki/Numeral_(linguistics)