Significatio vocis esum | Babel Free

Definitiones

1. accusative supine of edō
   Old-Latin, accusative, form-of, supine
2. nominative/accusative/vocative neuter singular
   Old-Latin, accusative, form-of, neuter, nominative, participle, singular, vocative
3. accusative masculine singular
   Old-Latin, accusative, form-of, masculine, participle, singular

Exempla

“Item male conferunt fui sum⟳ ero, quod fui est perfectum, cuius series sibi, ut debet, in omnibus partibus constat, quod est fueram fui fuero; de infectis sum⟳ quod nunc dicitur olim dicebatur esum et in omnibus personis constabat, quod dicebatur esum es est, eram eras erat, ero eris erit; sic huiusce modi cetera servare analogiam videbis.”

Likewise they are wrong to group fui/sum/ero together [and then complain of irregularity] because fui, being a perfect, is actually in its own series, which is internally consistent as expected: fueram/fui/fuero. Of the non-perfects, sum was in olden times esum, hence it used to be consistent in all persons: esum-es-est/eram-eras-erat/ero-eris-erit. You will find that all other verbs follow this pattern [of perfects agreeing with perfects, non-perfects with non-perfects].

Gradus CEFR