-aŭ

Unknown. -aŭ was the only possible vocalic ending left in the language, after the dedicated part-of-speech suffixes, apart from the uncommon -eŭ. There are, however, sporadic parallels in other languages, such as Lithuanian tačiau "however".

-aŭ

1. A neutral suffix that does not specify the part of speech of a word. Generally used for words such as hodiaŭ "today" which may be either an adverb or a conjunction, noun, preposition, or adjective, depending on context.

In theory the suffix -aŭ may be replaced by another grammatical suffix, or simply dropped, but in practice this does not occur outside poetry:

"aŭ does not belong to the root, but is only a conventional ending, and is thus as easily left off as the o in nouns." Zamenhof further wrote of the "neutral but definite ending aŭ", that is, that -aŭ does not define the part of speech of the root. He suggested in 1892 that it could be replaced with an apostrophe. —Kalocsay & Waringhien 1985:121.

-aŭ is not a productive suffix, and is only found with a few roots. These are:

adiaŭ (“goodbye”, interjection)

almenaŭ (“at least”, conjunction and adverb)

ambaŭ (“both”, adjective and adverb)

ankaŭ (“also”, adverb)

ankoraŭ (“still, yet”, conjunction and adverb)

anstataŭ (“instead of”, preposition and conjunction)

antaŭ (“before”, preposition) (the -aŭ cannot be removed)

apenaŭ (“barely”, conjunction and adverb)

baldaŭ (“soon”, adverb)

ĉirkaŭ (“around”, preposition and adverb)

graŭ (“because of”, preposition and neologism)

hieraŭ (“yesterday”, noun and adverb)

hodiaŭ (“today”, noun and adverb)

kontraŭ (“against”, preposition and adverb)

kvazaŭ (“as if”, conjunction and adverb)

malgraŭ (“despite”, preposition)

morgaŭ (“tomorrow”, noun and adverb)

preskaŭ (“almost”, adverb)

Of the adverbs, only baldaŭ (“soon”) can be used in the comparative. The preposition laŭ (“along”) and the numeral naŭ (“nine”) are bare roots, and do not contain this suffix.