Spanish Diphthongization

Spanish diphthongization is distinct from that of other languages. The Latin e and o often become ie and ue. This process is common in stressed syllables.

The word dental (dental) has an unstressed e. Compare this to the word diente (tooth), which is stressed on the first syllable. The word dental preserves the e of  Latin.

Here is a comparison of Spanish words with those of other Romance languages:

door

Spanish puerta Catalan porta French porte Italian porta Portuguese porta

fire

Spanish fuego Catalan foc French feu Italian fuoco Portuguese fogo

rock

Spanish piedra Catalan pedra French pierre Italian pietra Portuguese pedra

snow 

Spanish nieve Catalan neu French neige Italian neve Portuguese neve

Only Spanish has the diphthong ue in the words for door and fire. However, Italian also has a diphthong in the word for fire. Besides Spanish, French and Italian also have the diphthong ie in the word for rock. However, none of the other Romance languages have this diphthong in the word for snow, but Catalan has the diphthong eu. The diphthongs ie and ue are especially common in Spanish.