But I've lived my entire childhood in Canada, and I've been told by my friends and family that I make a lot of grammar mistakes when I speak Arabic. So I guess English is my strongest language.
Try making "grammatical mistakes" similar to the ones in the Qur'an, and see if they will correct you. If they do, insist that you are correct, and then later on show them specific verses from the Qur'an.
I remember the user "Al Razi" saying that he managed to deconvert one or two Muslims by making such deliberate mistakes.