Created attachment 58844 [details]
No Complain About "I can understanding English."
Type "I can understanding English."
No blue wave line appear.
I prove the grammar checker is on by typing "I'm an boy." and see blue wave line under "an boy".
The attachment is screenshot of the issue.
By saying typing, I mean type in LibreOffice Writer.
Created attachment 58845 [details]
Microsoft Word Does Complain
XiaoJun, I'm happy to report to you that with the LanguageTool (www.languagetool.org) extension, you would get a blue wavy line for "understanding"! (In current inbuilt LightProof doesn't do much.)