We have seen and heard that there are 3 different pronunciations for final -ed. It can sound like a t, d, or -ed. Is there any explanation for these different pronunciations and are there rules to help us know the right pronunciation?

There is an explanation. The -ed is affected by the sound that comes before it. Linguists have special categories for all the sounds in language, based on how the sounds are made and the position of the mouth and tongue. It is not necessary to understand everything about these categories but only to know the 3 categories and the sounds in them. The three categories that we will be working with are:

Voiceless sounds (sounds made without vibration of the vocal chords)

Voiced sounds (sounds made with vibration of the vocal chords)

Dental stops (sounds made by putting the tongue against the teeth)

It is important to understand that we are talking about sounds and not letters. These two should not be confused. In English , for example, we can make an f sound with the letters f (food), ph (philosophy), and gh (enough). Also, one letter may have more than one sound, such as the letter c in the words can, since, and ciao.