This is an activity that students do in pairs. One partner has a list of the present tense verbs which s/he must change to the preterite. The other student has the correct answers (this partner tells the other if his/her answers are correct and must also guide the other partner to the correct answer when necessary). When student get to the bottom of the page, they must switch papers and do the activity again. In my class I require students to repeat the activity until I instruct them to stop.
To use this activity, print enough copies of the first page for 1/2 of your largest class. Cut each page in half. Put students in pairs giving each person in the pair half of the sheet.
You can have students continue the practice by having them do the activity individually as homework. Use the blank copies on page two of the document for this purpose.