sometimes i cant edit sql developer

1It sounds like your accelerators might be confused and it no
2longer knows that the Backspace is the delete previous character
3key (the key mapping seems to get confused ocassionally).
4Go to Tools > Preferences > Accelerators and
5select "Delete Previous Char" in the Actions list and see if
6there is anything in the Accelerators on the right. If not,
7you can revert to the standard accelerators (which tends to
8fix the problem) by selecting the "Load Preset ..." button.