For regular verbs, you must add -ed to the base form or infinitive verb (or -d if the root form already ends in an e). The infinitive or base form of the verb is how the verb appears in the dictionary, for example, “walk”

[ verb] + [ed] = regular past tense verb

Irregular verbs, like “to be” for example, don’t follow a pattern or rule like regular verbs and so just have to be memorized.