In writing and enter is entered I get a double space that can't be reversed.

In writing a line to the end you automatically get a single space down. Fine. In writing a short line and enter is entered you get a double space down. Only in starting a new paragraph is this acceptable. In no other case is this acceptable. I find no way that an unwanted double space can be reversed which is unacceptable. I have been using Thunderbird for many years and always had single spacing . What must be done to have all single spacing?

                                                                                                  Thankyou