Spanish is a grammatically inflected language, which means that many words are modified ("marked") in small ways, usually at the end, according to their changing functions. Verbs are marked for tense, aspect, mood, person, and number (resulting in up to fifty conjugated forms per verb). Nouns follow a two-gender system and are marked for number. Personal pronouns are inflected for person, number, gender (including a residual neuter), and a very reduced case system; the Spanish pronominal system represents a simplification of the ancestral Latin system. Spanish was the first of the European vernaculars to have a grammar treatise, , published in 1492 by the Andalusian philologist Antonio de Nebrija and presented to Queen Isabella of Castile at Salamanca. The (RAE, Royal Spanish Academy) traditionally dictates the normative rules of the Spanish language, as well as its orthography. Differences between formal varieties of Peninsular and American Spanish are remarkably few, and someone who has learned the language in one area will generally have no difficulties of communication in the other; however, pronunciation does vary, as well as grammar and vocabulary. Recently published comprehensive Spanish reference grammars in English include , , and . Verbs Every Spanish verb belongs to one of three form classes, characterized by the infinitive ending: -ar, -er, or -ir—sometimes called the first, second, and third conjugations, respectively. A Spanish verb has nine indicative tenses with more-or-less direct English equivalents: the present tense ('I walk'), the preterite ('I walked'), the imperfect ('I was walking' or 'I used to walk'), the present perfect ('I have walked'), the past perfect — also called the pluperfect ('I had walked'), the future ('I will walk'), the future perfect ('I will have walked'), the conditional simple ('I would walk') and the conditional perfect ('I would have walked'). In most dialects, each tense has six potential forms, varying for first, second, or third person and for singular or plural number. In the second person, Spanish maintains the so-called "T–V distinction" between familiar and formal modes of address. The formal second-person pronouns (, ) take third-person verb forms. The second-person familiar plural is expressed in most of Spain with the pronoun and its characteristic verb forms (e.g., 'you [pl.] eat'), while in Latin American Spanish it merges with the formal second-person plural (e.g., ). Thus is used as both the formal and familiar second-person pronoun in Latin America. In many areas of Latin America (especially