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