There was a discussion on the SoftMaker Office support forum about grammar checkers. SoftMaker TextMaker has a spell checker, but not a grammar checker. In response to a request from users that they add one, the SoftMaker team responded with a link to this: https://www.antidote.info/en It looks interesting. I am attempting to get more information about it. It looks like it should work with any word processor, so that would include Libre Office Writer, Open Office Writer, TextMaker, and I guess anything else. It may even be better than the brain-dead grammar checker that Microsoft bundles in Word. At least for now, this is just a heads up, not a recommendation. If I do buy it and try it, I'll post a report.
I have learned that the single user license for Antidote 10 (the "perpetual" license) allows installation on up to three devices for personal use, one device for business use. It installs as an additional tab in Word's ribbon menu. For other word processors, Antidote has to be run externally.
