Most Old English verbs happen in two classes, the Weak Verbs and the Strong Verbs. A smaller number of verbs are the irregular Verbs of Being and Present-Preterite Verbs.
Weak verbs shift from present to preterite tense by adding a suffix, usually with a d-consonant. Modern English verbs such as jump/jumped and skip/skipped still follow this pattern.
Strong verbs shift from present to preterite tense by means of internal vowel shifts, called ablaut shifts in modern linguistics. Modern English verbs such as think/thought and eat/ate still follow this pattern.