Read first what is the topic all about. Reply what is really in your heart...express it through a good grammar and quality post. Select word which is common and do not used unfamiliar word. Just be your self in answering the topic.
Yeah, some people using very unfamiliar words just to say that they are good in english, which is not required to do, what important is try to construct an answer that is easy to understand but it should be informative also.