Just like in English, some verbs in Latin just don't follow the rules of other verbs. These are called irregular verbs. Here are the irregular verbs in Latin.
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Sum, Esse, Fui, Futurus: to be

This verb is one of the most commonly used verbs in Latin, so it is VERY important to know. You have to memorize the conjugations of the imperfect tenses (present, imperfect and future) because they are irregular. The perfect tenses, however, are conjugated normally, so you just need to know the third principal part (fui). Here are the conjugations of the present, imperfect, and future forms. 
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Present

​	Singular	Plural
1st person	sum	sumus
2nd person	es	estis
3rd person	est	sunt