You searched for: “ablaut”
ablaut (s) (noun), ablauts (pl)
The change of one root vowel into another one to show a variation of tense, part of speech, or meaning: The following ablauts show how certain verbs change their tenses: swim, swam, swum; sing, sang, sung; drink, drank, drunk; sink, sank, sunk; and many other irregular verbs.
The students were asked to find the single example of an ablaut in the poem that they memorized.
The grammar teacher wrote several examples of ablauts on the chalkboard for the students to study.