Hi Samantha
 
Just say "to be " is a special verb you have to learn it by heart
 
then you can introduce regular and irregular verbs 
 
You can find very good worksheets explaining the Simple Past with 
 
 
a drawing                        +     verb  + ed 
(showing a person
= I , you ....)
 
then you can find the same for the negative form and the interrogative form
 
you can use the same for the irregular verb 
 
explain that the form of the  irregular verbs have to be learnt  by heart, too
 
I did it this week with two classes and it worked quite well
 
after having explained the grammar practice with many games to learn the new forms
 
go step by step 
 
Good luck  and MERRY CHRISTMAS
 
Swissprof
