Here's why:
* Regular Verbs: Many verbs form their past tense and past participle by adding "-ed" (e.g., walk - walked, play - played). These are called "regular verbs".
* Irregular Verbs: A significant number of verbs have irregular forms (e.g., go - went, see - saw, eat - ate). These verbs don't follow the "-ed" rule.
* Special Cases: Some verbs ending in "-y" change the "y" to "i" before adding "-ed" (e.g., study - studied).
So, to summarize:
* You can add "-ed" to regular verbs to form the past tense and past participle.
* You cannot add "-ed" to irregular verbs.
* Some verbs have specific rules for forming the past tense and past participle.