웃다 conjugated in the past tense is 웃었어요
There is a rule that says if the verb stem ends in a ㅅ then you drop the ㅅ if the conjugation being added starts with a vowel. (For example, 짓다 -> 지었어요)
Why is 웃다 an exception? How do you recognize that a word is an exception to this rule?