Lately I’ve been looking into the origins of the Spanish irregular verbs oh-so-affectionately called “boot” verbs. They are more properly called “stem-changing verbs” because their final stem vowel changes from e to ie (e.g. negar/niego), from o to ue (e.g. poder/puedo), or from e to i (e.g. medir/mido). It’s exciting to discover that this verb class is a perfect example of the two classic forces in language change: sound change and analogy. As any basic linguistics textbook will tell you, sound change affects all words with a given sound, and analogy then messes things up.
Two sound changes are responsible for the “boot” verbs: the change of Latin short /ĕ/ and /ŏ/ to the diphthongs (vowel sequences) /ie/ and /ue/ in stressed syllables, and the raising of /e/ to /i/ before the sound /j/, which is pronounced like English y. The first change is more common because it affected -ar, -er, and -ir verbs. It’s the change behind the form of much Spanish vocabulary, including such common words as fiesta and puerta, from Latin festa and porta (see this previous post). And because it’s confined to stressed syllables, it’s the source of the classic “boot” pattern, where the diphthong occurs in the singular and the 3rd person plural (pUEdo, pUEdes, pUEde, pUEden) but not in the nosotros and vosotros forms, where stress falls on the verb ending (podEmos, podÉIs).
Analogy messed up this tidy result by turning some regular verbs into boot verbs and some boot verbs into regulars. The former is akin to the emergence of dove as an alternative to dived, by analogy to drove and other irregular “strong” English verbs. The most common verb that “went boot” is pensar, which shouldn’t have a stem change because its /e/ comes from a Latin long vowel. Some examples of former boot verbs that are now regular are prestar (formerly priesto, priestas, etc.) and diezmar (the original infinitive was dezmar).
The raising of /e/ to /i/ before /j/ only happened in a few verb forms, but analogy took it the rest of the way. The /j/ that triggered this change occurred in Latin -īre verb endings; this is why all Spanish verbs in this boot class are -ir verbs. For example, the –iō ending of Latin mētiō “I measure” came to be pronounced /jo/ in Vulgar Latin, triggering the change of /met/ to /mit/. This vowel change then spread, by analogy, throughout the full “boot”, and the /j/ was eventually lost. (The /t/ also turned into a /d/, obviously.)
Spanish was actually supposed to have four types of boot verbs, because /j/ affected /o/ as well as /e/, raising it to /u/ in a number of -ir verbs. However, in these cases analogy truly ran rampant and /u/ completely took over the verb, changing Latin mollire, for example, to Spanish mullir “to hoe”. No modern forms of this verb reflect the original /o/. The same thing happened to subir. Its Latin source was sŭbīre; without the /j/ the short /ŭ/ of the stem would have turned into an /o/, i.e. sobir.
For more, see pp. 156-161 In the 1991 edition of Ralph Penny’s marvelous A History of the Spanish Language (unfortunately, not the edition pictured below, which has eliminated some detail.)
I wrote again about boot verbs a few days later, in this post.