Check to make sure it's not using British spelling.
Here's a help page to step you through the process.
Thanks, mark. That is the page that I went to before asking for help, but it doesn’t help me. I checked to make sure I was set on American English.
This is so odd, as I have been using Opera for a few years now, and, until this problem showed up, I never even looked at that settings page. So I can’t imagine what was changed and how it was changed.
Right now, as I type this, every single word is showing as misspelled. And if I right-click for a “correction” the phrase “no spelling suggestions” is not even dark like the other choices in the menu.
As an experiment, I just right-clicked on the word “click” in the previous paragraph and then selected “Add to dictionary” from the menu. So, now, that word is the only one among all of the words in this box that is not underlined as misspelled!