Although you can tell by looking at nearly any Spanish word how it is pronounced, the reverse isn't always true. Because of the silent h and the existence of letter pairs that can sound alike, creating homophones, it is often possible to match more than one spelling with a particular sound.
This is especially true in the case of the b and v, which, except in a few types of nonstandard speech, share the same sounds. Native speakers frequently mix up the letters in their writing, and there are a few words (such as ceviche or cebiche, a type of seafood dish) that can be spelled with either letter. If you hear a word with the b/v sound, here is a guide to making an educated guess about which letter to use in spelling:
Most English cognates use the same letter as English does: Most of the cognates of Spanish and English are derived from Latin, and generally the use of b or v remains unchanged. Examples include botella (bottle), batalla (battle), avisar (to warn, related to "advise"), vocabulario (vocabulary), vibrar (vibrate), versátil (versatile) and vicio (vice). (Throughout this lesson, the definitions given aren't the only ones possible.) Exceptions: Among the words that don't follow this rule are verbs related to probar (to try, related to "probe"); words relted to gobernar (to govern); some verbs ending in -bir such as recibir (to receive), concebir (to conceive) and percibir (to perceive); alcoba (bedroom, related to "alcove"); and haber (to have).
Words with prefixes: Although several much-used prefixes use a b, the only common prefix using a v is vice- (meaning "instead of"), as in vicepresidente (vice president) or vicerroy (viceroy). Prefixes using b include ab- (indicating negation or separation), bi- (two), sub- (under), bene- (good) and bio- (life). Examples of such words are bilingüe (bilingual), subordinar (to subordinate), abstinencia (abstinence), bendito (blessed) and biología (biology).
Suffixes: Common suffixes with a b include -bilidad and -ble, both of which suggest having a quality of some sort. Examples include culpabilidad (guilt), amabilidad (kindness), terrible (horrific) and amable (friendly). There are also -fobia, indicating a fear, such as with claustrofobia (fear of closed spaces) and entomofobia (fear of insects). The most common suffix with a v is -ivo, which indicates having a certain quality, such as in activo (active) and pasivo (passive).
With m and n: B can follow m and v can follow n, but the opposite is extremely rare. The sound of -mb- and -nv- are identical. Examples include envasador (packer), embajador (ambassador), enviar (to send), cambio (change), también (also), ambiente (environment), inversor (investor) and envidiar (to envy).
Preceding r and l: The b can come before either of these consonants, although v cannot. Examples include posible (possible), hablar (to speak), broma (joke), abrazo (arm), abril (April) and obligar (to require).
In verb conjugations: Conjugations of the imperfect tense use a b, as in comprábamos (we were buying) and hablabas (you were speaking). Three verbs andar (to walk), estar (to be) and tener use a v in the preterite tense. Examples: anduve (I walked), estuviste (you were) and tuvieron (they had).
Quick exercise for review: To review this material, read the brief selection from the first chapter of Don Quijote de la Mancha by Miguel Cervantes on the following page and see how many times you can apply one of the above rules to the spellings of the words.