Every word is a particular part of speech. Which part of speech has the fewest words?
Going by the lists at English Grammar Revolution, there are slightly fewer Conjunctions than Pronouns in English. Both manage at least three dozen, though some are phrases rather than individual words.
Next up would be Interjections, but this is a looser category, open to creative expression. Gadzooks!
Some grammarians would argue that certain elements are syncategorematic meaning they are effectively their own part of speech. In other words there are no other words like them. Contenders for this category might be the word there in existential constructions such as:
- There's an elephant over there.
... or the infinitival marker to, as in to perambulate.
However, many grammars include these words in other parts of speech. For example, in the Cambridge Grammar of the English Language, they consider there to be a Pronoun, and infinitival to to be a Subordinator.
Of course, how many parts of speech there are in a category also depends on how many categories are recognized by that particular grammar. For example, in some grammars Pronouns are their own part of speech, in others they are a subcategory of the Noun class. Generally, word categories that are considered to be ‘lexical’, ( - in other words Nouns, Verbs, Adjectives and Adverbs), are generally larger than the grammatical ones (prepositions, determiners and so forth). Probably the smallest part of speech category posited by a well-established, and academically recognized grammar would be the Subordinator category in the internationally renowned Cambridge Grammar of the English Language (Huddleston & Pullum 2002).
CaGEL reassigned a large number of what older traditional grammars used to call Subordinating Conjunctions to the Preposition class. This left a very small number of true Subordinators. In fact there are only six! In case you're interested in which words are included in the class, they are:
- that, for, whether, infinitival-to, interrogative-if and how.
Edit note: I originally wrote why for how here. Sorry!