Programmatically generating incorrect grammar from correct sentences must be really tough. There are so many more ways to incorrectly structure a sentence than

Programmatically generating incorrect grammar from correct sentences must be really tough. There are so many more ways to incorrectly structure a sentence than there are correct ones. Random idea: what happens if you use Google translate to generate the incorrect sentence, I.e. Translate it to other languages and then back again. If the resulting sentence doesn't match the original, add it to the dataset.