A preterite (/ˈpretəraɪt/) 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 miscellaneous spelling changes). For irregular verbs, there might be no change, or there might be a vowel change. When listing the forms of an irregular verb, it is customary to list the preterite second, between the base form and the past participle, e.g. "give + gave + given."
Full modal verbs have preterites too, e.g. "could" is the preterite of "can".
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".