Since the most common way to create the past tense and past participle in English is to add -ed to the base form of the verb, English learners will see these words all the time. What many of my students have struggled with is knowing how to pronounce them.
To understand the pronunciation rules, first you have to understand the difference between vibration sounds and air sounds. (Many resources will also refer to these as "voiced" and "voiceless," with "voiced" meaning vibration and "voiceless" meaning air).
So how do we know if a word ends with a vibration sound or an air sound? When you finish saying a word, notice if the last sound creates a vibration in your mouth, or if it has a hard, quick noise you make and can't hold. Words like "buzz," "plan," or "ram" will make a vibration. Go ahead and say them. See how you can hold out the "z" sound in "buzz," the "n" sound in "plan," and the "m" sound in "ram?" Well, that's vibration! Now try saying the words "cook," "stop," and "laugh." Notice that the "k" sound in "cook," the "p" sound in "stop," and the "f" sound in "laugh" are made by pushing air out of your mouth rather than vibrating. Those are air sounds.
Now let's make these words past tense by adding the -ed endings and see how we would say them. The important thing to remember is that vibration goes with vibration and that air goes with air. "D" is a vibration sound, and "T" is an air sound.
Let's start with the vibration sounds. The words "buzz," "plan," and "ram" all have one syllable, ending with a vibration sound. When you put the -ed endings on these, they will still have one syllable, but you'll add a "d" sound. "Buzzed," "planned," and "rammed" will have one syllable and the vibrating "d" at the end, since vibration goes with vibration. Make sure you're not adding syllables!
If a word ends with a vowel sound, that will always indicate vibration. The "o" sound at the end of "follow" means that "followed" will still have two syllables, just like "follow," but with the "d" sound.
Now let's take a look at the air sounds. The words "cook," "stop," and "laugh" also all have one syllable each. When you put endings on these, they will also still only have one syllable, but for these, you'll add the "t" sound. "Cooked," "stopped," and "laughed" will have one syllable and the air-sound "t" at the end, since air goes with air. Again, make sure you're not adding syllables! The temptation is to say "stop-ped," but it should be more like "stopt."
So, if you're adding "t" or "d" sounds, what can you do if a word already ends with a "t" or "d" sound? In these cases, you will need to add an extra syllable, which will sound like "id." Let's take a look at the words "need" and "want." Both of these words have one syllable in their base forms, but when we add -ed, we'll need to put the extra syllable "id" to make the sounds properly. "Needed" and "wanted," therefore, will both have two syllables. This is the only time we put extra syllables for an -ed ending.
There are a few exceptions to these rules. The words "aged," "dogged," "ragged," "blessed," "learned," "wicked," "crooked," "naked," and "wretched" will all have the extra syllable "id" sound when used as adjectives. However, remember the three ways to pronounce -ed: "t" for air, "d" for vibration, and "id" for t and d, and you'll be on your way to much better pronunciation!