I know it's meant to be short, but I find "Grammatical problem" too general. Hope all rules with this short message can benefit from clearer (though still relatively short) messages.
E.g. for id="PROGRESSIVE_VERBS", the existing long message is
This verb is normally not used in the progressive form. Try a simple form.instead.
My suggestion for the short message:
Verb not usually in progressive form
It's a brief (newspaper headline) version of the long message. With this, the user may understand the reason for flagging right there, instead of clicking further.