Irregular verb “sein” (to be) and other irregular verbs
A number of verbs in the present tense second- and third-person singular forms (“du” and “er/sie/es”) have a stem vowel change. In the case of “essen” and “sprechen” above, they substitute “i” for “e” (as do geben, nehmen, and treffen). Other substitutions include “ä” for “a” (laufen, fahren, waschen) and “ie” for “e” (lesen, sehen, empfehlen).
In addition to stem vowel changes, many irregular verbs have very different simple past forms and past participles, and are categorized based on seven similar patterns of vowel changes, which is discussed under the past tense sections.