Spanish is an inflected language. The verbs are conjugated in conventional form, with present, simple past, imperfect, future, conditional, and the perfect tenses. Each person (first, second third) and number (singular, plural) carries its own inflection. Nouns come in two genders. There are a couple of neuter demonstrative pronouns. Adjectives must agree with the nouns they modify in gender and number. Nouns are not further inflected according to their grammatical role in the sentence. Pronouns, however, come in nominative (subject), accusative (object) and dative (indirect object) versions.
Like the other Romance languages, the word order is normally Subject-Verb-Object (with several common variations for special emphasis) and adjectives usually follow the nouns they modify. Subject pronouns are omitted when not required for clarity of meaning.
Most noun plural forms are regular. Most nouns ending in "a" can be reliably regarded as feminine except words of Greek origin (such as poema, idiota, dilema), which are masculine. "Día" is masculine, as it comes from a fifth declension (masculine) Latin noun, not a first declension feminine noun. Nouns ending in "o" are reliably masculine, except for "mano," which comes not from a second declension (masculine) Latin noun, but from a feminine, fourth declension noun. Most neuter nouns in Latin entered Spanish as masculine. Words ending in "-ión" are from Latin's third declension, and are now always feminine in Spanish. Words ending in "-aje" (like viaje and bagaje) are masculine in Spanish, but feminine in Portuguese. Likewise, many words that in other Romance languages are of one gender, in Spanish crop up as the other. Puente and Leche are two examples. The first is masculine, like French but unlike Portuguese, and the second is feminine, unlike both French and Portuguese, where it is masculine.
The formation of diminutives (usually -ita on the root) and augmentatives (usually -ón or -ona on the root) is also highly regular, though a few variants do appear (as in "cajetilla" instead of "cajita" for a pack (or box) of cigarettes in Mexico). In Costa Rica the -tico ending is used, and to such an extent that "tico" has become a (not particularly flattering) nickname for a person from Costa Rica. The diminutives and augmentatives not only alter the dimension of the noun thus modified; in most cases they either change the meaning entirely or add an additional connotation. Most often diminutives make nouns more agreeable and less threatening, and augmentatives accomplish the reverse. The most often used diminutive is probably "ahorita" for "right away" - a diminutive of "ahora," meaning "now." It is designed to make the hearer think that less time will be involved. This is often not true, and an "ahorita" can take much longer to carry through than an "ahora." Another common example is "soltera," which means a single woman, and "solterona," which means an old maid.
Most commonly used Spanish verbs are irregular. The most common form of verb irregularity involves orthographic changing of the stem (as in "quiero" from "querer" or "pienso" from "pensar." Another common irregularity is with the first person singular indicative, which is the root for other verb formations. In the case of "saber" it was shortened to "sé." "Haber" is similar, though its third person form has been elongated from "ha" to "hay." The first person singular indicative of "ir" is "voy." A very common form is the irregular preterit, mainly inherited from Vulgar Latin. Examples include "traje" and "dije" for "traer" and "decir." As in Latin, "ser" and "ir" are highly irregular, and they share a common preterit, "fui-fuiste-fue-fuimos-fuisteis-fueron." The use of the tenses generally follows the practices of Vulgar Latin and other Romance languages. For example, the present tense can often be used instead of the future when there is no need to emphasize future time. Thus "me voy" can mean "I go away" and also, "I shall go away," depending on the context.
Like other Romance languages, Spanish makes great use of the reflexive. This is often employed as a sort of passive voice without the formality, as in "se quebró" - "it broke." It also conveys the vague actor, which in English is "they," as in "they said that..." In Spanish it is "se dice que...".
Imperatives and subjunctives are regular, formed from the preterit stem (regular or not), and are employed in much the same way as in other Romance languages. Unlike Portuguese, which has maintained the future subjunctive, Spanish just uses the present. For example, in the expression "sea lo que sea" (let it be what will be), the second "sea" refers to the future, and technically should be in the future subjunctive. In Portuguese the same expression is written "seja o que for."
Spanish uses both the verb "ser" (from Latin essere) and the verb "estar" (from Latin stare), as verbs for "to be." The first is reserved for permanent or inherent traits, while the second is a verb of physical placement or temporary condition. It is not always obvious to a foreign speaker when each verb is used. Moreover, the rules in Spanish are a bit different from those used in Italian and Portuguese.
Spanish requires a "personal a" - the use of the preposition "a" ("to") before any direct object that is a person. It does not apply to entities, just to people. For example, "pick up the car" will be "recoger el coche" in most places, but "pick up the teacher" will be "recoger a la maestra."
The grammar, vocabulary and syntax of Spanish is defined, maintained and adjudicated by the Royal Spanish Academy in Spain (called the "RAE"). Similar language academies exist in 21 other Spanish-speaking countries.