The correct definition of the prefix semi- is “half”, as it portrays the idea that the patient was only partially conscious. Another example of semi- is a semiformal dance. It is more formal than casual clothes, but you are not expected to wear a tuxedo.
The prefix meaning “before” is pre-. → A movie prequel is a movie that takes place before the original.
There are several prefixes meaning “not” such as un-, dis-, or a-. → If you are undecided it means you have not made a decision.
The prefix meaning “after” is post-. → A postgame show is a broadcast after a game is finished.