The first part of the trim command asks you to select a cutting edge, then push enter in response to select objects on the command line.
This finish select objects routine is essential with the trim command because you might want more than one cutting edge.
Routing a cable that runs inside a house to around the outside for example, would require Using both outer walls to be used as cutting edges.
Once you have pushed enter to finish objects, the command line will ask you to select objects to be trimmed.
There's a very useful short cut to Trim.
After you have entered the Trim command and when you are asked to select cutting edge, rush right mouse instead.
Note that the select cutting edge prompt on the command line is skipped.
Just go to your drawing and click away. You will trim any trimmable object back to it's next available cutting edge.
Note that only objects which are crossed by another object can be trimmed. So in these cases you should use the erase command.