First, try to watch this video in which an English teacher explains how to pronounce the -ed endings.
Now, if you click here, you will be able to read about the pronunciation of the -ed ending.

You are ready to do some exercises to distinguish the three different pronunciations of the past simple in English:
EXERCISE 1
EXERCISE 2
(In this exercise, you need to LISTEN TO the pronunciation of some sentences. You can do it at home!!)
EXERCISES IN PAPER
(You can download the two previous exercises in paper, if you want to practise them at home, without using your computer)EXERCISE 3
EXERCISE 4
(You also need to listen to the verbs, but you can do the exercise without sound)EXERCISE 5
EXERCISE 6
EXERCISE 7
EXERCISE 8
EXERCISE 9
EXERCISE 10
EXERCISE 11