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I have a question for a machine translation exam which reads;

"Provide examples where unigram, brigram, trigram and 4-gram models would fail to capture a grammatical constraint of the English language."

Can anybody give examples of what grammatical constraints are as I don't have a clue...

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    In the sentence, "Mary, though she is very tired by the end of the day, slowly and carefully finishes all the assignments.", the agreement between Mary (or she, both singular) and finishes (verb calls for singular subject) cannot be handled by a 4-gram model because these words are more than 4 words apart. Is this what you had in mind? – prash Jun 4 '13 at 12:23
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A grammatical constraint is a "rule" that prohibits speakers from building a particular kind of sentence. For example, you can't say in standard English he go to school, or to he goes school, etc. What they are asking you (if I understand the question correctly) is to provide examples of these kinds of constraints that a n-gram based grammar would miss. An example that comes to mind is that of long distance relations and structural closeness. If you need specific examples I could give you a couple.

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  • For example: President Obama has visited Los Angeles is perfectly grammatical, but *President Nixon has visited Los Angeles is not. Because President Nixon is dead, and the perfect construction does not allow subjects that are both human and dead. It would take a very intuitive N-gram model to capture this. – jlawler May 22 '13 at 19:26
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    @jlawler: That's a semantic constraint. Nothing ungrammatical about it at all. Just like "unicorns are cool" doesn't become ungrammatical just because unicorns don't really exist. – hippietrail May 23 '13 at 4:16
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    @jlawler, so you can't say that a human "has died"? Sounds OK to me. – dainichi May 23 '13 at 6:16
  • The Hot News sense of the perfect is fine, but only if it's recent. And whether it's a grammatical constraint or a semantic one is quite irrelevant to what an Ngram-based grammar could recognize. Unless you want to define "grammatical constraint" as "anything an Ngram-based grammar could recognize". – jlawler May 23 '13 at 13:16
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    Many sentences in print about Nixon when he was alive became ungrammatical upon his death? I should quote Chomsky on grammar vs. semantics: "Colorful green ideas sleep furiously". – hippietrail May 23 '13 at 16:12

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