origins.latin.sound-changes
From Latin to Spanish: the sound changes that did the work
A handful of regular changes turn most Latin words into their Spanish forms: F- → h-, stressed Ĕ → ie and Ŏ → ue, -CT- → -ch-, PL-/CL-/FL- → ll-, softened -p- -t- -c-, -NN- → ñ.
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Explanation
Sound change is regular rather than random, which is what makes it usable. The main laws between Latin and Castilian: initial F- became an aspiration and then silence, spelled h- (FARINAM > harina, FACERE > hacer, FILIUM > hijo, FUMUM > humo), except before the diphthong ue, where it stayed (FOCUM > fuego, FORTEM > fuerte). Stressed short Ĕ and Ŏ broke into ie and ue (TERRAM > tierra, PETRAM > piedra, SEPTEM > siete, PORTAM > puerta, NOVEM > nueve, MORTEM > muerte). Between vowels, -P-, -T- and -C- softened (LUPUM > lobo, VITAM > vida, AMICUM > amigo, CATENAM > cadena).
The consonant clusters are just as systematic: -CT- became -ch- (NOCTEM > noche, OCTO > ocho, LACTEM > leche, FACTUM > hecho, DIRECTUM > derecho); initial PL-, CL- and FL- became ll- (PLUVIAM > lluvia, PLENUM > lleno, CLAMARE > llamar, CLAVEM > llave, FLAMMAM > llama); -LL- gave the letter ll and -NN- gave ñ (ANNUM > año, CANNAM > caña), joined by -MN- and -NI- (DOMNUM > dueño, HISPANIAM > España). Final -M and often final -E simply dropped (PANEM > pan, SOLEM > sol, MARE > mar).
The payoff for learners is that this explains the stem-changing verbs. Latin stress fell on the stem in some persons and on the ending in others, and Ĕ and Ŏ only diphthongized when stressed — so you get puedo but podemos, tiene but tenemos, pienso but pensamos, duermo but dormimos, cuenta but contamos. The irregularity is not caprice; it is a fossil of where the Latin accent sat. The same split shows in noun–adjective pairs: hueso but óseo, huevo but oval, tierra but terrestre.
Examples
I can't right now, but we can do it tomorrow.
Last night it rained and the iron door got wet.
The wolf drank all the milk.