Charles E. Sprague, Hand-book of Volapük — 13 (1887)
“ The word "do" is used in English as an auxiliary denoting emphasis, as, "I do believe." In Volapük this cannot be translated otherwise than "I believe."The same tense-vowels,a, ä, e, i, o, u,are used with some words other than verbs, when time is to be distinguished. ”
