Editing Esperanto grammar (section)

==Parts of speech==
The suffixes ''‑o'', ''‑a'', ''‑e'', and ''‑i'' indicate that a word is a [[noun]], [[adjective]], [[adverb]], and [[infinitive]] [[verb]], respectively. Many new words can be derived simply by changing these suffixes. Derivations from the word ''vidi'' (to see) are ''vida'' (visual), ''vide'' (visually), and ''vido'' (vision).

Each [[root word]] has an inherent [[part of speech]]: nominal, adjectival, verbal, or adverbial. These must be memorized explicitly and affect the use of the part-of-speech suffixes. With an adjectival or verbal root, the nominal suffix ''‑o'' indicates an abstraction: ''parolo'' (an act of speech, one's word) from the verbal root ''paroli'' (to speak); ''belo'' (beauty) from the adjectival root ''bela'' (beautiful); whereas with a noun, the nominal suffix simply indicates the noun. Nominal or verbal roots may likewise be modified with the adjectival suffix ''‑a'': ''reĝa'' (royal), from the nominal root ''reĝo'' (a king); ''parola'' (spoken). The various verbal endings mean ''to be [__]'' when added to an adjectival root: ''beli'' (to be beautiful); and with a nominal root they mean "to act as" the noun, "to use" the noun, etc., depending on the semantics of the root: ''reĝi'' (to reign). There are relatively few adverbial roots, so most words ending in ''-e'' are derived: ''bele'' (beautifully). Often with a nominal or verbal root, the English equivalent is a [[prepositional phrase]]: ''parole'' (by speech, orally); ''vide'' (by sight, visually); ''reĝe'' (like a king, royally).

The meanings of part-of-speech affixes depend on the inherent part of speech of the root they are applied to. For example, ''brosi'' (to brush) is based on a nominal root (and therefore listed in modern dictionaries under the entry ''bros'''o'''),'' whereas ''kombi'' (to comb) is based on a verbal root (and therefore listed under ''komb'''i''').'' Change the suffix to ''-o,'' and the similar meanings of ''brosi'' and ''kombi'' diverge: ''broso'' is a brush, the name of an instrument, whereas ''kombo'' is a combing, the name of an action. That is, changing verbal ''kombi'' (to comb) to a noun simply creates the name for the action; for the name of the tool, the suffix ''-ilo'' is used, which derives words for instruments from verbal roots: ''kombilo'' (a comb). On the other hand, changing the nominal root ''broso'' (a brush) to a verb gives the action associated with that noun, ''brosi'' (to brush). For the name of the action, the suffix ''-ado'' will change a derived verb back to a noun: ''brosado'' (a brushing). Similarly, an abstraction of a nominal root (changing it to an adjective and then back to a noun) requires the suffix ''-eco,'' as in ''infaneco'' (childhood), but an abstraction of an adjectival or verbal root merely requires the nominal ''-o: belo'' (beauty). Nevertheless, redundantly affixed forms such as ''beleco'' are acceptable and widely used.

A limited number of basic adverbs do not end with ''-e,'' but with an undefined part-of-speech ending ''-aŭ''. Not all words ending in ''-aŭ'' are adverbs, and most of the adverbs that end in ''-aŭ'' have other functions, such as ''hodiaŭ'' "today" [noun or adverb] or ''ankoraŭ'' "yet, still" [conjunction or adverb]. About a dozen other adverbs are bare roots, such as ''nun'' "now", ''tro'' "too, too much", not counting the adverbs among the [[Table of correlatives (Esperanto)|correlatives]]. (See [[special Esperanto adverbs]].)

The part-of-speech endings may double up.<ref>[https://bertilow.com/pmeg/vortfarado/principoj/frazetvortigo.html ''Plena Manlibro de Esperanta Gramatiko'']</ref> Apart from the ''-aŭ'' suffix, where adding a second part-of-speech ending is nearly universal, this happens only occasionally. For example, ''vivu!'' "viva!" (the volitive of ''vivi'' 'to live') has a nominal form ''vivuo'' (a cry of 'viva!') and a doubly verbal form ''vivui'' (to cry 'viva!').