The problem is (if I remember correctly) that if you want to check the grammar of a sentence you have to understand the sentence. The meaning and how a word is used determines if the chosen grammer is correct. Computers are rather bad at understanding natural language because it is very ambigious.

It's easy to come up with sentences that can put even humans on the wrong path, for instance: "Time flies like an arrow and fruit flies like a banana.". Here flies can be either a noun or a verb.

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