From Teflpedia

A preterite is the form of the verb used by itself in the past tense.

The preterite of a regular verb is formed by adding -ed (with miscellanous spelling changes). For irregular verbs, there might be no change, or there might be a vowel change.

Some full modal verbs have preterites too.

The preterite is not used with auxiliary do (preterite: did); rather a bare infinitive is used instead, as a second grammatical marker is not necessary. e.g. "I ate a sandwich" "I did eat a sandwich".