Questions & Answers about На завтрак я делаю бутерброд с колбасой и сыром.
На завтрак literally means “for breakfast.” In Russian, на + accusative is a common way to say “for (a meal or occasion).” You’re indicating purpose or timing (“what you eat for breakfast”).
Here завтрак is in the accusative singular because it’s the object of the preposition на. Since завтрак is a masculine inanimate noun, its accusative form is identical to its nominative form—no ending change.
Бутерброд is the direct object of the verb делаю (“I make”), so it must be in the accusative. It’s masculine and inanimate, so accusative = nominative (no -а ending). If it were animate (e.g. я вижу брата), it’d take a different form.