I recently used the following question on a quiz:

A gentleman ______ I had never seen before smiled at me.

The students were to put the relative pronoun in the first blank and then write whether it was a defining or non-defining relative clause in the blank after the sentence. They were also to add proper punctuation (commas) where necessary.

Some of the students are arguing with me about the answer to this question. Is it a non-defining relative clause or a defining relative clause? I have asked several teachers for their opinion and they all are in agreement that it is a non-defining relative clause because of the indefinite article in front of the noun in the main sentence and should be written as follows:

A gentleman, whom I had never seen before, smiled at me.

The students who are questioning the answer insist that it is defining because it is stating which gentleman smiled at me and, therefore, necessary for the meaning of the sentence.

Does an indefinite or definite article have anything to do with what type of relative clause it is? Please help. Thanks
The indefinite article certainly helps in the determination, though I doubt that it is much of a deciding point. In any case, your clause is most likely restrictive (defining) because the gentleman in this isolated sentence has no definition otherwise-- it is as simple as that.

Even with 'the', it could just as well be a defining clause in this isolated situation, as no other information exists about the gentleman or the situation.

A gentleman whom I had never seen before smiled at me. (completely new information)
The gentleman whom I had never seen before smiled at me. (There were 2 gentlemen-- one I knew and one I didn't know.)
A gentleman, whom (by the way) I had never seen before, smiled at me.
The gentleman (that I mentioned before), whom I had never seen before, smiled at me.

In this series, the first, restrictive (defining), case is the most likely for the indefinite article and the last, non-restrictive (non-defining), case is the most likely for the definite article.

All in all, not a good quiz question.
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To make it a defining clause you would need to use the relative pronoun 'that'. Using 'whom' makes it a non-defining clause.
I agree with MrM completely.

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