Using commas in relative clauses

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I’ve noticed that a couple of very proficient German students of mine are not sure when to use commas in long sentences in their English essays so today I am referring you all to a helpful page from that great site The OWL at Purdue University which talks about commas. There are a few rules, but I don’t think they are too complex. One of the simpler rules which students often forget about is the one about using commas with non-essential elements in a sentence, and not using commas with essential elements in a sentence.

Let’s have an example:
1. My sister, who works in IT, is coming to visit me tomorrow.
2. The person who is speaking is my sister.
In sentence 1 I hope you can see that the clause ‘who works in IT’ is not essential to the meaning of the sentence. It is just a bit of extra information about my sister, the important information is that she is coming to visit me. Because this is a non-essential element in the sentence we use commas (this is called a non-defining relative clause). In sentence 2 the clause ‘who is speaking’ is essential to the understanding of the sentence as it defines which person is my sister. It is an essential element in the sentence (called a defining relative clause). Take a look at this good page from Purdue and read about this and other uses of commas. Enjoy!