Irregular verbs, also called strong verbs in German, can be distinguished from the other types of verbs. In German, there are about 150 strong verbs. The vowel, which normally changes in the verb, is called “Ablaut”.
In English and in German, there are different kinds of verbs. What constitutes an irregular verb is if there are conjugation changes in either the simple present tense (das Präsens), or the simple past (das Präteritum/Imperfekt), that differ from the regular conjugation patterns. Verbs that show no changes in their conjugation in any of the tenses are called “regular verbs”. Verbs that have irregularities in their conjugation in any or all of the tenses are considered “irregular verbs”.
1. Regular verbs
To demonstrate this, let’s first look at a verb that shows no irregularities in its conjugation. This means that the word stem stays the same for all pronouns and all tenses. (Example: spielen – to play)
Notice that the verb stem “spiel-” stays the same for all pronouns and all tenses.
2. 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. Some verbs substitute “i” for “e” (e.g. treffen). Other substitutions include “ä” for “a” (fahren, waschen), “äu” for “au” (laufen) and “ie” for “e” (lesen, sehen, empfehlen). Some irregular verbs show no changes in the present tense conjugation, but do in the simple past conjugation (e.g. gehen).
Notice how in the example above, the vowel change appears for “du” and “er/sie/es”. The word stem is also different in the simple past conjugation.
The verb “sein” (to be), and “haben” (to have) are both irregular verbs, as they follow an irregular conjugation pattern.
3. Common Vowel Changes in the Present Tense
a -> ä
Examples: schlafen, tragen (also au -> äu: laufen, du läufst)
Ich schlafe, du schläfst, er/sie/es schläft, wir schlafen, ihr schlaft, sie/Sie schlafen
e -> i
Examples: geben, essen, helfen, sprechen
Ich gebe, du gibst, er/sie/es gibt, wir geben, ihr gebt, sie/Sie geben
e -> ie
Examples: sehen, lesen
Ich sehe, du siehst, er/sie/es sieht, wir sehen, ihr seht, sie/Sie sehen