Morphologically, adverbs resemble words that are usually assigned to other categories such as interjections, prepositions,subordinating and coordinating conjunctions. Their common characteristic is that they are all indeclinable, invariable, or whatever one wants to call it, that is, they do not belong to the inflectional nominal and verbal categories. A common denominator for words belonging to the classes mentioned, that is often found, is’ ‘particle'. In fact, ‘particle’ is as good as any other term as long as it is used consistently in a morphological sense, as it is by Matthews (1965; cf. 2.2.2.). Since in Latin linguistics (but also elsewhere; see e.g. Ruijgh 1971: 99 on Greek), the notion ‘particle’ is often used in syntactic sense as well, or rather in several syntactic senses, I will avoid it and use rather the term ‘invariable’ .

Actually, there are a number of syntactic (as well as semantic) differences between adverbs and other invariables, to which I will now turn. The first and most important distinguishing criterion that I will discuss is the criterion of ‘sentence-valence’ - I borrow the term from Van Wijk (19'67: 240-1). Adverbs are said to be capable of occurring independently, in one-word-sentences, whereas e.g. prepositions are said not to occur in this way. On the other hand, it has .been observed that interjections do not occur other than independently. I will try to show that this criterion is difficult to apply in Latin in the first place and, secondly, intersects with the traditional class of adverbs.

Adverbs as optionally free forms

The only Latin grammar, in which the sentence-valence criterion is used is O'Brien (1965). He observes (p. 39-40) that adverbs optionally occur as free forms, in contradistinction to interjections, which obligatorily occur in that way, and other invariables, which never occur as free forms. Kurytowicz (1964: 21), too, considers the occurrence of traditional adverbs as free forms the primary criterion by which they can be differentiated from prepositions.

Most words which may occur independently are not likely actually to occur in one-word-sentences unless in an appropriate setting of context and/or situation, e.g. only if an appropriate question precedes or, conversely, if an appropriate answer follows. These words may be said to have ‘restricted sentence-valence’ (Van Wijk 1967: 241).