(And I suppose the logical follow-up question is - are there any functions words take that are not included in these 5 categories?)
1) deliver specific content/ meaning
- adjective
- adverb
- noun
- verb
2) establish relationships between other words, phrases and parts of sentences
- conjunction
- wh-word
- pronoun
- prepositions
- copulas (link verbs)
3) modify or refine the meaning of another word, phrase or sentence
- adjective
- adverb
- determiner
- numeral
- negative
- noun
- preposition
- modals
4) frame a question
- wh-word
- modals
- be, do and have (??)
5) Establish verb tense and time reference
- adverbs
- nouns
- prepositions
- auxiliaries
- verb participles
Maybe interjections, particles, lexemes (but it seems that once these words find context in a sentence they would fall into one of the above functional categories)?