"My brother is lazy."
Translation:Tá mo dheartháir leisciúil.
Think about the copula as an equals sign. For the most part, you use is to link two nouns or pronouns together saying either what or who something or someone is. "Lazy" is a quality or state of being, so we use "ta." If you wanted to that "your brother = a lazy person" - stating the kind of person he is - then you would use the copula.