Adverbs, prepositions, and conjunctions
In describing Greek and Latin grammar, classical scholars dealt at length with those parts of speech that offered interesting inflectional paradigms: verbs, nouns, adjectives, and pronouns. Other words were consigned with little comment to three other categories. These were the class standing 'at the verb' (Latin adverbium), the class 'positioned before' others (prae-positiō), and the class for 'joining together' (con-jūnctiō). With rare exceptions, modern grammarians, linguists, and text writers have retained this focus on the "major" categories of V, N, Adj, and Pro, to the relative neglect of what are still called ADVERBS, PREPOSITIONS, and CONJUNCTIONS.
It is understandable why these last three categories have been overshadowed by the others. They are fewer in number, simpler in morphology, and apparently straightforward in their meanings and grammatical roles. Thus, aside from notes on por/para, pero/sino, and mood contrasts after conjunctions, these parts of speech are relegated to vocabulary lists in Spanish courses. However, errors such as the following suggest the need for more attention to them.
*Paré el coche porque del tráfico.
*Mi amiga nos esperaba afuera el restaurante.
*Escuchamos a la radio mientras estudiando.
*Es difícil a romper.
Given the usual vocabulary matchings tarde = late, porque = because, afuera = outside, mientras = while, a = to, with no further input or explanation from book or teacher, it is natural for students to generate the foregoing sentences on the basis of how the English words are used. Corrected in piecemeal fashion, such errors will continue because the problem is as much grammatical as it is lexical. In fact, in order to master the usage of Spanish adverbs, prepositions, and conjunctions, students require at least four kinds of information about them: lexical, categorial, semantic, and grammatical.