* "Estoy impresionado" is correct and means "I am impressed."
* "Donde esta mi semental" is grammatically correct but a bit strange in this context. It means "Where is my stallion?"
Possible interpretations:
* Are you trying to express your admiration for a horse? You could say: "Estoy impresionado con tu semental" (I am impressed with your stallion).
* Are you looking for your horse? You could say: "Estoy buscando mi semental, ¿dónde está?" (I am looking for my stallion, where is he?)
* Are you referring to a man you find attractive? It's best to avoid using the word "semental" to describe a man, as it is considered vulgar and offensive.
To get a more accurate translation, please provide more context or specify what you are trying to say.