AutoCad :: How To Navigate Around Program

I use Autocad 2011 to read and modify drawings supplied by my client for the design and installation of ducting. I was going along nicely although slowly due to my transition from Visual cad. These are two completely different program and Autocad seems incredibly complicated until you start to learn a few commands and figure out how to navigate around the program.

Anyway, I have had about 20 hours with the program and am on the bottom of the learning curve when for some reason, the method of working suddenly changed, I am not aware of making the changes and cannot see how to change back.

First off, if I drew a few lines and then wanted to delete them, I simply selected each line and pressed delete. This was the same for most commands, I would select the items and then click the command.

Now, I have to click the command, then select the items and press enter. The main problem with this is that to delete anything, I now have to select "cut" or "erase", select all of the items and then press enter.

What I want to do is select the items and press delete but the delete key no longer works. how to get back to using my delete key.