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John takes a long sling, connects its one end to the cam which is inserted into the hole close by her and connects the other end to the climbing rope.

There are many cams. "The cam" refers to a particular cam. I think I should use which but some grammar references says I have put a comma before which. I don't think it's correct to put a comma before which.

Should I use that or which?

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What is involved here is the distinction between restrictive and non-restrictive relative clauses.

A restrictive relative clause is called that because the clause restricts the scope of what is said about the noun or noun phrase it modifies. It specifies which noun is meant.

In your case, if which is inserted into the hole close by her is a restrictive clause it means that John connects the sling to a cam in that position and no other.

If it is a non-restrictive clause, then the position of the cam is irrelevant. It is just by happenstance that the cam is there.

The role of the comma is to distinguish these two sorts of clause: a non-restrictive clause is set off by a comma (or two, if it falls in the middle of the sentence), a restrictive clause is not.

RESTRICTIVE: John takes a long sling, connects its one end to the cam which is inserted into the hole close by her and connects the other end to the climbing rope.

NON-RESTRICTIVE: John takes a long sling, connects its one end to the cam, which is inserted into the hole close by her, and connects the other end to the climbing rope.

NOTE: I myself would put a comma after by her in both sentences, to make the structure clearer; but that is not required, and here it might confuse you, so I have left it out.

There is one other clue to whether the clause is restrictive or non-restrictive: if the relative pronoun which introduces the clause is that, the clause must be restrictive. The relative pronoun that cannot be used at the start of a non-restrictive clause. Who or which, however, can be used with either sort of clause.

For about the first sixty years of the 20th century it was fashionable among some "authorities" to require that at the start of all restrictive relative clauses; but that "rule" was never widely followed and is now disappearing just as fast as it arrived. Grammar Girl is one of those who maintains it. But if you are taking a class in English, do as your teacher tells you -- until you have passed all the exams!

NOTE: Here I give my reasons for almost never using that as a relative pronoun; but you are no more bound by my practise than by Grammar Girl's.

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  • As ever, it helps to remember English is primarily spoken. The "non-restrictive" sense is normally distinguished in speech by a pause (comma in the pallid imitation written form). Given that unambiguous mechanism, it's pretty much irrelevant whether you use that or a wh-form.
    – FumbleFingers
    May 15, 2013 at 3:53
  • @FumbleFingers Actually, in improvised speech (as opposed to read or practised speech), pauses are not diagnostic because a pause may fall at any point the speaker needs to slow down and formulate what they're going to say next. (And an intentional, calculated 'pause' is rarely an actual pause, it's just a lengthening and a falling tone on the previous vowel, with gemination of the terminal consonant.)
    – StoneyB on hiatus
    Jun 7, 2013 at 0:57
  • Sure, but as John L pointed out elsewhere, "pause" is a pretty inaccurate term, since what we perceive as pauses (doubtless much influenced by written forms) are mainly fluxions of intonation & stress. Which we're all really good at picking up on, even if English isn't formally classified as a "tonal" language.
    – FumbleFingers
    Jun 7, 2013 at 3:44
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    @Mari-LouA If I understand your question correctly, it's not even usually true. A relative clause can be used that way, but it is also used to restrict the reference of its head to an individual or subset of a previously identified group (e.g. "Here are six pennies; let's look more closely at the two which were minted before 2008").
    – StoneyB on hiatus
    Oct 18, 2016 at 18:29
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    @Mari-LouA Coming up with a better definition of the restrictive/non-restrictive contrast--and of modifier/predication contrasts in general--has occupied me a lot lately. My latest thought is that a modifier supplies either old assertions or presuppositions about the head, while predications supply new assertions about the head.
    – StoneyB on hiatus
    Oct 18, 2016 at 18:45

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