Hi Linda,
Thanks for posting your question here. As it sounds like you have most likely already noticed, pressing the "Enter" key while working in the description editor will produce a double space by default. If you would like to move down only one line, you just need to hold the "Shift" key while you press the "Enter" key.
I hope this helps. If you need any further assistance with your account, please feel free to contact our customer support team by mousing over the "Help" tab and selecting the "File Support Case" option.
-Mike