A common mistake that students make is this:
We didn't wanted to do homework
THIS IS WRONG. You only change "don't" to "didn't"
Another mistake students make is that they change "didn't" to something else. Some examples your teacher saw:
We doedn't want to do homework
We didedn't want to do homework
DIDN'T is always DIDN'T
I didn't want
you didn't want
he/she/it didn't want
they didn't want
you didn't want
we didn't want