Etymologiae, Book I, chapters 10-14: De adverbio to De interiectione
Chapter X. On the adverb The adverb is so called because it is added to verbs (verbis accedat), as in bene lege. Bene is the adverb, lege the verb. Hence it is called adverb because it is always joined to a verb and so completed. For the verb alone fills out the sense, as scribo. But the adverb without a verb does not have a full signification, as hodie. Add a verb to it, “today I write,” and with the verb joined you have filled out the sense.
Chapter XI. On the participle The participle is so called because it takes the parts of the noun and the verb, as if particapium. For from the noun it claims for itself genders and cases, from the verb tenses and significations, from both number and figure.
Chapter XII. On the conjunction The conjunction is so called because it joins senses and sentences. For by itself it is worth nothing, but in the coupling of speech it offers a kind of glue. For it either joins nouns, as “Augustine and Jerome,” or verbs, as “he writes and reads.” The force of all is one, whether they couple or disjoin. Copulative conjunctions are so called because they join sense or persons, as “you and I shall go to the forum.” The very word et has joined the sense. Disjunctive, because they disjoin things or persons, as “let me or you do it.” Subjunctive, because they are subjoined, as que. For we say regique hominique Deoque; we do not say que regi, que homini. Expletive, because they fill out the matter proposed, as “if you do not wish this, at least do that.” Common, because they are placed and subjoined anywhere, as “I shall therefore do this,” “this therefore I shall do.” Causal are so called from cause, because they think to do something, as “I kill him because he has gold”; that is the cause. Rational are so called from the reason which each uses in doing, as “how shall I kill him, lest I be recognized? with poison or with iron?”
Chapter XIII. On the preposition The preposition is so called because it is placed before (praeponatur) nouns and verbs. Accusative and ablative prepositions are named from the cases they serve. Inseparable ones (loquellares) are so called because they always cohere with words (loquellis), that is, verbs, and are worth nothing when set alone, as di, dis. Joined to verbs, they make the figure, as diduco, distraho.
Chapter XIV. On the interjection The interjection is so called because, thrown in (interiecta), that is, interposed in speech, it expresses the feeling of a moved mind, as when one exulting says vah, one grieving heu, one angry hem, one fearing ei. These sounds are proper to each language, and are not easily carried over into another.