All regular verbs in the past positive form end with -ed. But there are 3 major forms of pronunciation. If you don't say them correctly, Americans will think that you are talking about the present. It will be confusing. Some students have difficulty making the 'd' sound, and as hard as they try, they don't ever do it like a native speaker. But most students can do the 't' ending, with some practice.
The 3 -ed Pronunciations
-ed with t sound
the last sound of the verb is made by a consonant with air only
- picked
- laughed
- watched
- hoped
- kissed
- boxed
-ed with a d sound
the last sound of the verb is made with a vibration of the voice
- subbed
- hugged
- judged
- called
- dreamed
- cleaned
- remembered
- shaved
- mowed
- played
-ed with an 'id' sound
for verbs where the last sound is a 't' or a 'd'
- wanted
- invited
- recommended
- needed
If you're really interested in why this is, take a look at the examples more carefully. Pick has 3 sounds: 'p,' 'i,' and 'k'. The last sound is 'k'. Say the sound 'k'. Did you use your voice? No. All you used was air. Now, say the sound 't'. Did you use your voice? No. All you did was use air. So, air to air: 'k' to 't'. Together: 'kt'. The same is true of the other examples: laughed, watched, hoped, kissed, and boxed.
Now, let's look at hug. It has 3 sounds: 'h,' 'u,' and 'g'. 'G' is the last sound. Make it. Did you use your voice? Yes, you did. Now, try 'd'. You used your voice as well. Voice to voice: 'g' to 'd.' Together: 'gd.' This follows for the other words in the category.
If you're really brave and want to look at the last category, read on. Ok. Now, want, it has 4 sounds 'w,' 'a,' 'n,' and 't.' 'T' is the last sound. And 't' is made by air only. Following the rules above, it should be followed by another 't'. That would be crazy. It would sound like 'wantt!' We can't do that. So we say 'want-id'. The same is true for need. The last sound 'd' is made with a vibration of the voice. Ok, vibration letters are followed by 'd'. That also would be strange. You'd say 'needd'. That sounds terrible. So, again, we say 'need-id.'
Very, very complex and I promise you that no American knows these rules. Ask them why you pronounced -ed differently and they won't be able to tell you (unless they're an ESL teacher). Do you have to memorize these rules? Not really. If you memorize the basic lists above, then other words that end with the same letters will naturally take on the same rules. For example, if you've memorized the past pronunciation of 'kissed' as 'kisst', then when you come to 'missed' you'll naturally say 'misst' and not 'miss-id'.