从Manhattan论坛上搜到的Ron关于however用法的解释。我真是爱死他了!!!
the words "however" and "therefore" are not used in american english as they're used in british english.
in british english, these words are used as coordinating conjunctions, in the same way as are and, or, yet, and the like. in american english, on the other hand, you CANNOT use them as coordinating conjunctions; you have to use them in the same way as words/constructions such as nevertheless, despite all of these things, etc.
for instance:
i left home twenty minutes early, however i was still late. --> i think this is correct british usage, but it is WRONG WRONG WRONG in american english.
i left home twenty minutes early; however, i was still late. --> correct american usage. note the semicolon (which could also be a period, if these were longer sentences) and the comma, BOTH of which are essential.