Tools/Automatic Hyphenation is a document setting, both in TextMaker 2010 and 2012, and in Microsoft Word, too. If you want to get rid of hyphenation forever, open your default document template (usually Normal.tmv), turn hyphenation off, save it, and base future documents on it.
Thank you VERY much for this suggestion. I have now done what you suggest, and I think that solves the problem of having to remember to turn off Automatic Hyphenation with each new document.
[EDIT: Upon re-reading your post, I see that you talked of two consecutive dashes. TextMaker 2012 indeed puts the first dash on the old line, and the second dash on the following line. But TextMaker 2010 behaved exactly alike. But even Microsoft Word does this. Why don't you use real dashes? Ctrl+(NumPad-) gives you an en-dash, and Ctrl+Alt+(NumPad+) gives you an em-dash. Simulating long dashes with consecutive short dashes is bad, typographically speaking.
I never encountered the problem in TextMaker2010, but perhaps I simply never created a document with a dash at the end of a line. I'm very pleased to know about Ctrl+(NumPad-). I'll try to remember to use that (I might add that your instructions for creating an em-dash don't work on my computer, but I think the en-dash is really what I want). I do wish there were a setting in TextMaker to turn two hyphens into a dash. Before I used TextMaker, I used Microsoft Office, and I think I remember that Word would automatically turn -- into a solid dash, either by default or with a user-controlled setting, I don't recall which. I'm thus very much in the habit of typing -- and would welcome a similar setting in TextMaker. But now that I know about Ctrl+(NumPad-), that's a little less important, at least on my desktop with a numerical keypad. This wouldn't be so easy on my netbook, nor on many laptops.
I'm very grateful to you for your very helpful response. I'd be thrilled if you could also suggest solutions to the problems I've been having with TextMaker's mishandling of hyperlinks, as I explained in an earlier posting to the forum.