I have 11 sentences (I page per sentence so you can project on screen) with examples of problematic sentences in preterite vs imperfect. Many of the verbs are irregular as well and nost have at least two blanks for words.
I use this activity by numbering students off to group them and then each group has a white board and a marker with one writer per question. With each sentence the white board is passed to another member of the group. Students can hold up white board and teacher can keep score as well.