-ут/-ют are endings of the First Conjugation Group verbs in Plural Number, Third Person form in Present and Future..
See all possible forms of verb "нападать" (to attack) here.
Russian words tend to change endings according to the role they play in the sentence. It is called inflections.
Inflections ot nouns, adjectives, numerals, pronouns and like are called Declensions and are defined by the Cases of the language.
Inflections of verbs are called Conjugations and have nothing to do with Cases.
Normally if your mother tongue is not so inflective as Russian you need to study much theory and train hard before you can use that mechanism (unless you are a little child and able to learn languages intuitively).