@JTT,
AAgain, a too simplistic answer. Sometimes the borrowed word describes something the language doesn't cover. Sometimes the borrowed word is just an alternate word for the same thing. sometimes it's thought of as a tonier word to use for a less high-toned native word. Sometimes it's a euphemism, or sometimes it's new so it's thought to have more punch, e.g. cojones for balls, or testicles for balls (borrowing for both reasons).
Sometimes it follows the rules of the borrowing language, sometimes not for long periods of time. Alumnus. alumni, alumna, alumnae would be an example where the borrowed word has persisted for several centuries with the original plurals intact.
Beware of over-generalization, JTT. Tsarstepan is right.. Grammar geeks are the most humorless of the lot
