In this article we are going to offer you the different rules to use the letters b and v in Spanish, because it's usually a great confusion for the foreign students that learn Spanish as a foreign language. The following explanation has been extracted of Gramática Española de J. Alonso del Río.

I - It is written with B:

1.The words that in their origin language (Latin, Arab, etc.) have b or p. They can be grouped for example: bien, bueno, bondad, bendecir, benemérito, beato, biblia, biblioteca and all ones related etymologically to them, as well as these ones formed with the prefix bi-, that means two, like bigamia, bipolar.. By their etymology they are written with b for example: cabeza, obispo, lobo, and a lot of words that the use will indicate us, although we do not know their origin.

2.Words with the beginning bu-, bur- and bus-, like bufanda, burla, busto; and those that begin with sub-, like subasta and subalterno.

3.Words with the ending-bundo, -bunda and -bilidad, like tremebundo, moribunda and amabilidad. It's an exception movilidad and their compounds civilidad.

4.All the tenses of the verbs deber, beber and sorber and their compounds.

5.All the tenses of verbs whose infinitives end with -aber, like haber, saber and caber; with -bir, like escribir, recibir, and with -buir, like distribuir. The only exceptions are precaver, hervir, servir and vivir, and their compounds.

6.The endings -ba, -bas, -bamos, -bais and -ban of the imperfect tense of indicative corresponding to the verbs of the first conjugation, like lloraba, estudiabas, danzábamos, mirabais, fumaban.

7.The imperfect tense of indicative of ir: iba, ibas, etc..

8.The final letter of the word that ends with the sound b, like Jacob, baobab...

9.The syllables bra, bre, bri, bro,bru, bla, ble, bli, blo, blu and any case in which the sound b precedes to other consonant Ejs.: bloque, desbrozar, obnubilar, obstrucción.

II - It is written with V:

1.The words whose origin or etymology requires it, likeconvivencia, valenciano, párvulo. It can be grouped here the compound words formed with the prefix vice- (that indicates that a person can do the same sometimes that another one) like vicealmirante, vicepresidente; the names of geographic places that begin with villa-, villar-, vila-, like Villafranca, Villalba, Villarcayo, Vilaboa, and also the compounds words with valle-, vall- or val-, like Vallehermoso, Valparaíso, Valladolid.

2.Words that begin with the syllable ad, cla, di, pri, followed by its sound like adversidad, clavel, diversión, privilegio, etc. It's an exception dibujo and its derivatives.

3.Ending words in -viro, -vira and in -ívoro, ívora, like triunviro, Elvira, carnívoro, omnívora. It's and exception víbora.

4.Adjectives that end in: -ava; -ave; -avo; -eva; -eve; -evo; -iva; -ivo like the following examples:
dozava; suave; octavo; nueva; nieve; longevo; diva; superlativo. It's an exceptionárabe and sílaba and their compounds.

5.The tenses of the verbs whose infinitives have neither b nor v. Ejs.: voy, ve, vais, vayamos, etc., of the verb ir; anduve, anduviste, of the verb andar; estuve, estuviste, of estar; tuve, tuvieron, of tener, etc.

6.The verbs precaver, hervir, servir and vivir
and their respectives compounds, desvivir, convivir, sobrevivir, etc. and that ones with then ending -servar, like conservar.