I've just found this Twitter-Posting of some English-speaking guy struggeling with the German language. To be honest, German is not one of the easier languages to learn. Spelling and grammar is difficult but if you really mean it, you can actually do it!
In the above case:
"Umfahren" (running something over): The word "fahren" means of course driving and "um" describes the object (that you crash your car in) falling over (for example a tree). So the German "um" is similar to the word "over" in falling over. What us Germans do is to condense the event "Somebody crashes his vehicle into sth. and it falls over." in one word: "umfahren". If you are English-speaking and want to use that concept in your language you can establish the new verb "to falloverdrive" ;-D
"Umfahren" (drive around something): Yet again another language condensation. The "um" in "umfahren" is the short version of the german verb "drumherum" which means "around". If you want to establish a new verb in English you could say "to arounddrive".
Germans can differentiate both words by the context they appear in and by the pronounciation. In the first case (running something over) the operative word (the word with an emphasis on) is "um" and in the second case (drive around something) the operative word is "fahren".
Not that easy but not difficult enough to be an excuse for quitting your German lessons! :-)
Now tell me in the comments: Did you get it?