Why is there a comma in this sentence?

Jack has been studying zebras since 1972, when he started the famous Animal Center.

Isn’t the first clause independent, and isn’t when a subordinating conjunction making the second clause dependent? In this case, my basic grammar book says there’s no need for comma. Where am I wrong?

The explanation in my book says “the comma is appropriate between the noun ‘1972’ and the dependent, relative clause that begins with ‘when’.”

  • 4
    It's a non-restrictive clause, so it takes a comma. – Peter Shor Jun 26 '18 at 0:49
  • @PeterShor Thank you, I've looked up the definition of restrictive and non-restrictive clauses. However, I am not sure if those are sub-categories of dependent clauses, or if those two terms have nothing to do with dependent and independent. – Mike Jun 26 '18 at 5:35
  • There seems to me to be a natural break in the sentence which calls for a comma. You could quite plausibly begin a new sentence. 'That was the year he started...' – Kate Bunting Jun 26 '18 at 6:44

They are independant clauses. 1972 was the year he started studying zebras and 1972 was also the year he started the Animal Centre. It could easily have been two sentences but because it's the same year, a comma is used to add information about the year mentioned earlier.

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  • You are correct that the sentence could be split in two, but when he started the famous Animal Center is not an independent clause and would need to be reworded to be grammatical. – choster Jun 26 '18 at 14:45
  • is it not independant if the two clauses do not rely on each other? – esu Jun 26 '18 at 19:44
  • As I stated, the second clause is a dependent clause. – choster Jun 26 '18 at 21:23
  • Jack has been studying zebras since 1972. Jack started the famous Animal Center in 1972. Why are they dependant? – esu Jun 29 '18 at 14:17
  • You cannot split the example sentence in two simply by cleaving it in two, i.e. Jack has been studying zebras since 1972. (*When) he started the famous Animal Center. The second clause is dependent on the first; without it, when he started the famous Animal Center is simply a noun phrase. You would need to rephrase, minimally by deleting when to make it grammatical, but more thoroughly to retain the meaning—just as you have done in your comment. – choster Jun 29 '18 at 14:58

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