Excel - Selecting range removes formula...

Hi!

I need help, this is driving me crazy. I enter a formula on a cell (i.e. "=sum(" ), then when I try to select a range, it deletes the formula and the cell becomes the range (i.e. E12:E17). I been looking around web, keyboard settings, Excel general settings, etc. I cannot figure it out. Closest, not useful, answer so far has been "press F8". 

Thank you.