One language error that is often seen, not only in high school essays across the USA but in many places that should possibly know better, is the confusion of the two words imply and infer. Many think these two words mean essentially the same thing, but in fact the words are almost antonyms.
Put simply, you can imply an idea or opinion, but you infer an idea or opinion from something. Therefore, the person doing the implying is not the person doing the inferring!
Here's an example:
Jack's smile implied that he was having a good time.
Jack's mother inferred from Jack's smile that he was having a good time.