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I can't find any grammar reference to the correct usage of a specifying adjective clause. Consider an example:

The physical activity, so vital for the developing body, is often overlooked by the child's parents.

Is it grammatically correct? Am I right to surround the clause by the commas?

Thanking you in advance for any help.

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The sentence is grammatically correct, but the definite article at the start seems a bit suspect to me - unless the writer is referring back to an already specified type of physical activity. – Shoe May 23 '12 at 18:57
Shoe, in my view, it represents all physical activities in general as a class. Would it be better then to write "Physical activities, ..., are ...."? It seems identical to me, though. – neek May 23 '12 at 19:51
The article use here would depend on prior context; if exercise had been mentioned before as one of several things, then the article would be desirable. If not, not. – John Lawler May 23 '12 at 19:55
up vote 5 down vote accepted

This is a reduced relative clause, not a special construction with so. And the comma's necessary.

  • ... physical activity, so vital for the developing body, is often overlooked ...

is shortened by ordinary Whiz-Deletion from

  • ... physical activity, which is so vital for the developing body, is often overlooked ...

Since this is a parenthetical (nonrestrictive, supplementary) relative clause -- the kind that has to use which, and is ungrammatical with that -- the comma is necessary in order to indicate the intonation contour that distinguishes it from descriptive (restrictive, integrated) relative clauses in speech.

On the other hand, many would deprecate the use of so as an intensifier here; it marks the sentence as somewhat florid, reminiscent of advertising. In other words, it sounds like handwaving, done to avoid saying exactly how vital exercise is, thus presenting a claim without evidence for it.

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I'm now clear on this point. Thank you for such a comprehensive answer! – neek May 23 '12 at 19:39

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