Verbs that display ablaut in English, and that do not form their preterites with a dental suffix like -ed or added -t or d, are called strong verbs. There used to be several regular classes of strong verbs in English, and many more of them; virtually all monosyllable verbs were strong verbs in Old English. Now, there are fewer of them; the force of analogy has remade many of them in the image of weak verbs, those verbs that form the preterite with a dental suffix. Sound changes like the Great Vowel Shift have also obscured some of the underlying regularity of the former classes of strong verbs. Now most of them are considered irregular verbs.
Ablaut is a common characteristic of many Indo-European languages and is also known as gradation. Latin displays ablaut in verbs such as ago (present tense), "I drive"; egi, (perfect tense), "I drove." Ablaut is a semi-regular phenomenon that affects whole classes of verbs in Ancient Greek and Sanskrit.
Indo-European had a characteristic general ablaut sequence that contrasted the vowel phonemes o/e/ə/Ø through the same root. Most philologists believe that the presence of laryngeals in the Indo-European roots, and their subsequent loss in most daughter languages, led to the development of several parallel ablaut sequences in Indo-European and its daughter languages. When ablaut is a regular feature of a language's grammar, it is often called vowel gradation.
The ablaut is distinguished from the phonetic influence of a succeeding vowel, called umlaut.
See also: reduplication; augment