Hand-book of Volapük/11
FORMATION OF ADVERBS 
Numeral adverbs, expressing repetition, are formed by adding na. Kilna, three times ; kilidna, the third time.
Adverbs are formed from adjectivs, and sometimes from other parts of speech by adding o, corresponding to the English -ly.
Vifik, quick ; vifiko, quickly. Neit, night ; neitiko, nocturnally. When the adverb ending in iko immediately follows the verb, o is sometimes omitted ;
gonom vifik (vifiko), he runs fast (swiftly).
It is much clearer, however, to retain the o.