Complex question. I try to explain you the main uses.... Three ways. . 1.- Usually with Indirect Object. Yo compro flores a mi hermana (also para mi hermana)/ Me lo dijo a mi.. 2.- With Direct Object: Only with persons (or anímate things....)Like your sentences. Veo A un hombre y A una mujer (Man and woman are DO, or the turtle in the other sentence). But we don't use with unanimate things: Veo una montaña, veo una casa) 3.- Like direction to.....Voy a casa.