The -j- produced umlaut of the stem vowel in languages other than Gothic.

Regular strong verbs were all conjugated roughly the same, with the main differences being in the stem vowel.

Verbs other than form I have consistent stem vowels.

Certain verbs change their stem vowel for the second and third person singular forms.

Certain verbs change their stem vowels for the preterite indicative and subjunctive.

In modal verbs, the stem vowel will change for all conjugations of the singular simple present.

The stem vowel here is the vowel before the last consonant.

This happens when the stem vowel receives the stress.

Some strong verbs change their stem vowel in the second and third person singular of the indicative mood of the present tense.

The U-umlaut occurs when a stem vowel a changes to ö because of a u in the next syllable.