filling in and moving around a Numbers Sheet

Sometimes I hit the wrong keys; I apparently hit something that now makes odd choices when I use the Tab key. There is a bright green frame around the "current" cell, the cell is hilighted in bright green (i don't use green for any of my highlights), sometimes the space key does not work in the cell unless i use Option first, or backspace to delete a letter. If I try to leave a cell blank, the curser won't move, but the highlight does. The green frame might start trying to choose a different sheet tab. It's all over the place. It's just squirrely. It also does the same thing in sheets from other files. So it's Numbers specific, not just this file. Has anyone seen this before, and how do I get rid of it? I found no instructions for resetting the program to original. And I really don't want to use Time machine to go back, I've added too much info to track. Hope you can help. Thank you.