When teaching the past tense affirmative of the short form, point out that it is similar to the て-form. Start by writing two examples of verbs in the て-form, then write the short form next to it. Underline the different endings:
食べて(tabete) → 食べた(tabeta)
読んで(yonde) → 読んだ(yonda)
Explain that the て/で changes to た/だto make the short form present affirmative. Do the same with the negative past short form, but explain the い changes to かった. For example:
食べない(tabenai) → 食べなかった(tabenakatta)
読まない(yomanai) → 読まなかった(yomanakatta)
Note the two irregular verbs: 行く (iku) → 行った(itta) and いい (ii) → よかった (yokatta) , よくなかった (yokunakatta)
Test comprehension by making a worksheet that has both negative and affirmative conjugations. Alternate between the short form and て-form, and have students write out the other form. For example, if the question has 読んで, the student should write 読んだ.