A limited number of Esperanto

Esperanto

is the most widely spoken constructed international auxiliary language. Its name derives from Doktoro Esperanto , the pseudonym under which L. L. Zamenhof published the first book detailing Esperanto, the Unua Libro, in 1887...

 adverb

Adverb

An adverb is a part of speech that modifies verbs or any part of speech other than a noun . Adverbs can modify verbs, adjectives , clauses, sentences, and other adverbs....

s do not end with the regular adverbial ending -e. Many of these function as more than just adverbs, such as hodiaŭ "today" [noun or adverb] and ankoraŭ "yet, still" [conjunction or adverb]. Others are part of the correlative system, and will not be repeated here.

It should be mentioned that the class of 'adverb' is not well defined in any language, and that it is sometimes difficult to say whether a word is an adverb or not. The -e suffix is restricted in Esperanto for cases that are clearly adverbial.

Adverbs and the suffix -aŭ

Alongside its dedicated part-of-speech suffixes, such as adverbial -e, adjectival -a, and nominal -o, Esperanto has a grammatically neutral suffix -aŭ, which has no defined part of speech. Words ending in -aŭ may be used for multiple grammatical functions. They are typically words that in other languages people have difficulty identifying the part of speech. This suffix is not lexically productive, but is limited to a score of words which Zamenhof created with it. To specify the part of speech of these words, the dedicated part-of-speech suffixes may be added to the -aŭ. For example, anstataŭ "instead of" (preposition and conjunction

Grammatical conjunction

In grammar, a conjunction is a part of speech that connects two words, sentences, phrases or clauses together. A discourse connective is a conjunction joining sentences. This definition may overlap with that of other parts of speech, so what constitutes a "conjunction" must be defined for each...

) is the base of the adverb anstataŭe "instead", the adjective anstataŭa "ad interim, deputy", the verb anstataŭi "to take the place of", and the noun anstataŭo "replacement, substitution". Most -aŭ words have inherent adverbial uses; anstataŭ is one of the few that does not. The adverbial -aŭ words are,

Esperanto	English	parts of speech
almenaŭ	at least	conjunction and adverb
ambaŭ	both	adjective and adverb
ankaŭ	also	adverb
ankoraŭ	still, yet	conjunction and adverb
apenaŭ	barely	adverb
baldaŭ	soon	adverb
ĉirkaŭ	around	preposition and adverb
hieraŭ	yesterday	noun and adverb
hodiaŭ	today	noun and adverb
kvazaŭ	as if	conjunction and adverb
morgaŭ	tomorrow	noun and adverb
preskaŭ	almost	adverb

Theoretically, the -aŭ does not belong to the root, and may be dropped, or replaced by rather than suffixed by other grammatical suffixes. For example, alongside anstataŭ and anstataŭe there are anstat’ and anstate, but this is rare and in practice does not occur outside poetry. Kalocsay & Waringhien (1985) sum it up thus,

In 1892 Zamenhof proposed the following change: "Instead of the ending '-aŭ' in various words one can use an apostrophe [e.g. ankor’, apen’]... This will increase sonorousness, while not introducing any confusion (because '-aŭ' does not belong to the root, but is only a conditional ending, and thus as easy to leave off as the '-o' of the noun)." But this elision of '-aŭ' from adverbs was not put into practice; only in recent years have a few poets attempted to use it in verse. (p. 121; examples from p 409, note )

There have already been attempts to use these words in simple adverbial form (morge, apene, anke, ankore, almene, etc.), which would certainly bring along the simple adjectival form (ankora, almena, apena) and the nominal form (hiero, hodio, morgo). These attempts are even officially tolerated by the Academy [since 1910]. ... (p 409, note )

Bare-root adverbs

Other adverbs occur as bare roots; the distinction between bare-root and -aŭ adverbs is lexical. However, some of the bare-root words may be argued to be grammatical particle

Grammatical particle

In grammar, a particle is a function word that does not belong to any of the inflected grammatical word classes . It is a catch-all term for a heterogeneous set of words and terms that lack a precise lexical definition...

s and not true adverbs, in which case true adverbs may be derived from them by adding the ending -e. They are,

Esperanto	English	notes
for	away	cf. the derived adverb fore
jam	already, yet
ĵus	just now
nun	now	noun and adverb
nur	only
plej	most	cf. the derived adverb pleje
pli	more	pronoun and adverb; cf. the derived adverb plie
plu	beyond, further	cf. the derived adverb plue
tre	very
tro	overly, too much	cf. the derived adverb troe
tuj	at once, immediately

Occasionally in poetry forms such as nune and tree are seen, but this is considered grammatically redundant (Kalocsay & Waringhien 1985:409).