Learn Spanish (Latin America): Él viste bien.

The verb is vestir (to dress). It belongs to a special group of stem-changing verbs where the e in the middle changes to an i in the present tense for most forms (I, you, he, she, they). Because of this rule, we say viste instead of "veste".

Why does Él have an accent mark over the E?

The accent mark tells us that él means "he". Without the accent mark, el means "the". We need the accent here because we are saying "He dresses well."

Could I say Él viste bueno instead of bien?

No, you need to use bien. Bien means "well" and describes how an action is done. Bueno means "good" and describes a noun (a person, place, or thing). Just like in English, you say "He dresses well," not "He dresses good."