Wikipedia introduces inflection as

a word is modified to express different grammatical categories such as tense, case, voice, aspect, person, number, gender, mood, animacy, and definiteness.

Apparently whether or not -ly is an inflectional or derivational suffix can be debated.

If it were an inflectional suffix, which grammatical category is it related to?

As far as I can tell, -ly only changes the place in the sentence the word is allowed to be. Or what category of word it is allowed to modify (nouns or adjectives+verbs). I don't know what grammatical property is related to place-in-sentence.

2 Answers 2


What happens when -ly is added is that the part of speech of the resulting word changes and the word becomes an adverb (with some exceptions like 'deadly', 'friendly', etc. which are adjectives). So hypothetically you can consider the part of speech an inflectional category in which a word is turned into an adverb by adding -ly and into other parts of speech by adding other affixes.

But here we face a conceptual contradiction: in linguistics, language is subdivided into tiers (phonology, morphology, lexicology, etc.) which are arranged hierarchically, that is, an item belonging to each tier consists of items of the tier one level lower, and an item cannot consist of the items of the same tier: a syllable consists of phonemes, but a syllable cannot consist of syllables. Since a part of speech is a category of words that have similar grammatical properties, including similar morphological behavior in that they undergo inflection for similar properties, it means a part of speech is a set of inflectional categories. If we consider a part of speech an inflectional category itself we have an inflectional category which consists of inflectional categories, an entity consisting of entities of its own tier, which is forbidden by the logic of linguistics. So the part of speech cannot be an inflectional category. If you acknowledge the existence of the parts of speech, then -ly is not an inflectional suffix, at least in the “adj. → adv.” meaning. Q.E.D.

  • If -ly turned it into an adverb, wouldn't it be derivation and not inflection. Apparently there are two possible explanations of -ly : a derivational one, which your first sentence summarises; and an inflectional one, which my question is asking about.
    – minseong
    Oct 17, 2022 at 18:11
  • @theonlygusti - It can be either this or that, but not both at the same time. In my answer I prove it can be only derivational, it cannot be inflectional if you already consider it derivational. Your assumption is wrong, that -ly cannot be anything but derivational.
    – Yellow Sky
    Oct 17, 2022 at 18:15
  • If -ly were inflectional, it would not change the POS of course. So "slowly" would still be an adjective. The whole category "adverb" may no longer have to exist. Acknowledging this I don't think your proof works?
    – minseong
    Oct 17, 2022 at 19:37
  • @theonlygusti - Alright, no problem. But you haven't specified the initial state of things, the premises on which your grammar is based. My proof is based on the standard, most typical grammar, so your grammar diverges from the standard one in a way. If slowly is an adj., how can it modify verbs? Only if adjectives can modify verbs in your grammar, but then, it follows that parts of speech are not present in your grammar, and that's what I mentioned in my answer: if the PsOS exist, -ly can't be inflectional, it's derivational.
    – Yellow Sky
    Oct 17, 2022 at 20:51
  • 1
    @YellowSky Why does it follow that parts of speech are not present? Adjectives can arguably modify verbs in German just fine, and German certainly has parts of speech regardless. Adverbs are generally characterised as being able to modify verbs, adjectives and other adverbs in English; why would it not be possible to similarly characterise adjectives as being able to modify nouns, verbs and other adjectives (including adjectives inflected with the -ly desinence)? Oct 18, 2022 at 14:27

"tense, case, voice, aspect, person, number, gender, mood, animacy, and definiteness" are just examples, not an exhaustive list of what inflection can indicate.

If -ly is considered to be an inflectional suffix, and we go with the common (but arguably false) assumption that inflectional suffixes do not change a word's part of speech, then red and redly would both be adjective forms: the first used predicatively or attributively, the second used "adverbially" (the concept/terminology of "adverbial" is also a bit problematic, but I don't know enough to be more precise).

I don't know of a name for that inflectional category.

  • Do you know of any real languages that have inflection for changing whether something is predicative/attributive vs anything else?
    – minseong
    Oct 18, 2022 at 13:14
  • @theonlygusti: Russian has something called "long" and "short" adjectives, mentioned in Yellow Sky's answer to Are there any languages that mark predicative and attributive adjectives differently? Oct 18, 2022 at 15:06
  • @brasstacks - But in Russian both kinds of adjectives can be used as a predicative. The real difference is twofold: 1) the “short” ones can't be used as attributes; 2) as a predicative, the “long” ones mean a permanent quality while the “short” ones mean a temporary state, like in Он весёлый vs. Он весел “He's a merry person” vs. “He's merry right now”. Quite often the “long” ones are used in both senses, the “short” adjectives seem to be in the process of dying out.
    – Yellow Sky
    Oct 19, 2022 at 8:56

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