Spanish grammar requires agreement between nouns and their modifiers (noun-adjective agreement), between verbs and their subjects (subject-verb agreement), and between pronouns and the noun to which they refer (pronoun agreement).
In Spanish every noun has one of 2 genders: it may be masculine (lápiz, sapo, amigo) or feminine (pluma, rana, amiga). Most countable nouns have different forms depending on whether they are singular or plural (lápiz/lápices, sapo/sapos, amigo/amigos, amiga/amigas), but some countable nouns whose singular form ends with the letter 's' do not: for example, the names of the weekdays el/los lunes, el/los martes, el/los miércoles, el/los jueves and el/los viernes, but not el sábado/los sábados or el domingo/los domingos.
Adjectives modify nouns. Many adjectives vary for both gender and number (rojo/roja/rojos/rojas), many adjectives vary only for number (verde/verdes), and a few adjectives are invariable regardless of gender and number (vino). Adjectives that vary either for number or for both gender and number must agree in number or in gender and number with their nouns.
Most Spanish verbs have between 46 and 48 distinct simple forms. Most of them specify a particular subject identified by the combination of person (first, second or third) and number (singular or plural). These verb forms must agree with their subjects in both person and number.
Second-person verb forms and their associated subject and object pronouns (tú/vos, vosotros/vosotras, te, os) are used only when addressing the listener informally, such as when talking with a close friend or family member. When talking to strangers, to authority figures, or to anyone to whom one should show courtesy and respect one uses the formal subject pronouns usted and ustedes, third-person object pronouns (lo, la, los, las, le, les and se), and third-person verb forms.
There is a great deal of regional variation regarding whom one should address informally/familiarly/intimately and whom one should address formally/politely.
- In general, most of Spain requires this distintion for both singular and plural second-person address, using tú to address one person and vosotros/vosotras to address a group when one is expected or required to address someone familiarly/informally/intimately and using usted to address one person and ustedes to address a group when one is expected or required to address someone formally/politely.
- Parts of southern Spain and most American varieties require this distintion only when addressing one person. In these varieties only third-person plural verbs and the subject pronoun ustedes is used to address a group. When addressing one person familarly/informally/intimately one uses either tú or vos and corresponding verb forms, and when addressing one person formally/politely one uses usted and third-person singular verb forms.
- A few areas distinguish three levels of formality: these areas use usted and third-person singular verbs for formal/polite address, tú and its verb forms for an intermediate level of familiarity/informality, and vos and its verb forms for an intimate level of familiarity.
- A few areas of America do not use second-person verbs at all: to address one person one uses usted and third-person singular verbs, and to address a group one uses ustedes and third-person plural verbs.
Spanish has a variety of different pronouns. While some pronouns are invariable in form (alguien, nadie, yo, tú, vos), some pronouns vary according to number (quien/quienes), to both gender and number (él, ella, ellos, ellas, nosotros, nosotras, vosotros, vosotras), and occasionally to gender, number and person (possessive adjectives/pronouns such as mío/mía/míos/mías and tuyo/tuya/tuyos/tuyas).