What makes the Latin verb sum, esse, "to be," an irregular verb?

Question

Most Latin verbs follow some regular pattern in their conjugated forms, but not sum, esse...

Consider for example some regular 1st and 2nd conjugation verbs:

The 1st conjugation Latin verb laudo, laudare, "to praise," has the letter a between its verb root (laud-) and the personal endings (i.e. laud-a-s = laudas, "you praise"), and the 2nd conjugation verb moneo, monere, "to warn, advise," has the letter e in the same place (i.e. mon-e-s = mones, "you warn"). These regular patterns are helpful for memorizing the different conjugated verb forms.

But with sum, esse, there is no clear pattern! Even the verb root appears to change between the conjugated forms (i.e. su-m / e-s / es-t / su-mus / es-tis / su-nt)...

Answer 1

Sum, esse, "to be," is indeed considered an irregular verb, in that it underwent certain morphological and phonetic changes which produced differences in its present tense conjugated forms.

This is actually typical of the verb "to be" in most languages, since it is among any language's oldest verbs and thus is prone to changes over long periods of time!

However, the original formation of sum, esse followed a fairly regular pattern, even if that pattern became obscured in the end by its odd morphology – a term meaning “shape” or “form,” and referring to its final conjugated forms.

Here is what happened in the transition from Old Latin to Classical Latin (around the 4th century BCE), to produce the irregular forms of sum, esse:

1.    A “theme” vowel (e/o) was added to the end of the verb stem (es-) in those forms where the personal marker starts with a nasal consonant sound (m/n)...

2.    The initial e in the verb stem, es-, disappeared at the same time wherever the theme vowel was added...

3.    The theme vowels e/o (also common to Greek and other ancient Indo-European languages) transformed into the letters i/u respectively, a phenomenon which occurs in some of the personal markers themselves (see the table below), and in many other places in Classical Latin:

1st singular: es-o-m > _s-u-m > sum 1st plural: es-o-mos > _s-u-mus > sumus

2nd singular: es-s > e_-s > es 2nd plural: es-tes > es-tis > estis

3rd singular: es-t > est 3rd plural: es-o-nt > _s-u-nt > sunt

Some Important observations:

1. Note that of the possible theme vowels (e/o), only o is added to the 1st person singular and plural, and the 3rd person plural forms. This was for euphony, or “good sound,” since Latin speakers tended not to produce in their speech the combined sounds of s and m/n...

2. In all other places, the conjugated verb forms are lacking a theme vowel (which functions as a “stem vowel,” connecting the verb root to the personal marker), and so are said to be “athematic.” This difference in its conjugated forms, some being “thematic” and others “athematic,” is primarily what is meant by the verb being irregular...

3. In the 2nd person singular (e-s), when the s of the verb stem met with the personal marker -s, a contraction of the two identical letters (es-s) resulted in the disappearance of es-s, leaving behind the initial e- of the verb stem and the personal marker -s.