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Every time when I write a sentence in Microsoft Word like this:

the rats which are the food of snake are crazily growing.

It always recommends me to rewrite like this:

the rats, which are the food of snake, are crazily growing.

Is it true that which should always start with a comma?

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How could you trust the grammatical reliability of Microsoft programmers, who use the words "datas" & "schemas" when actually, the plural for schema is schemata, and data is already the plural of datum? –  Blessed Geek Apr 4 '13 at 1:23
    
@BlessedGeek: It is current accepted practice in AmE that, despite the source languages, 'data' is a mass noun (has no plural) so yes you're right that 'datas' is crazy but for the wrong reason, and 'schemas' is an acceptable alternative to schemata. –  Mitch Apr 4 '13 at 17:05
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marked as duplicate by FumbleFingers, Robusto, Matt Эллен, Rory Alsop, Mitch Apr 4 '13 at 17:05

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3 Answers

up vote 6 down vote accepted

No, it is not true. This is a bug in Microsoft Word, just one out of millions. You really should not take it too seriously.

What is correct is that if there is no comma before the which, that it is a defining relative clause, but if there is a comma, it is a non-defining one.

  1. The rats which are the snake's food are growing crazily. (defining)
  2. The rats, which are the snake's food, are growing crazily. (non-defining)
  3. The rats that are the snake's food are growing crazily. (defining)
  4. The rats (which are the snake's food) are growing crazily. (non-defining)
  5. The rats are growing crazily.

Numbers 1 and 3 mean exactly the same thing.

Numbers 2 and 4 mean the same thing as each other, and are essentially the same as number 5.

This is discussed in this answer to ELU question #96.

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Turn off the MS Word grammar checker. It is totally bugged and of no use. Unless you enjoy not understanding what's going on. –  John Lawler Apr 3 '13 at 18:03
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Short answer - no. You would not use a comma in the following examples, though MS might advise you to do so:

Do you know which one you want?

I'll let John choose which one to have.

He always knew which cafe had the best food.

As a basic rule, if you can remove the which clause and still have a complete sentence (tchrist's example 4) then it would be best to enclose it in commas.

However, you have to be sure of the meaning you are trying to convey - context is everything. For example:

The chairs, which I had painted blue, were very comfortable. [The chairs were very comfortable. I had painted them blue.]

The chairs which I had painted blue were very comfortable. [As opposed to some other chairs, which were not very comfortable.]

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The simple answer:

If you're talking about a specific kind of thing you must use comma. look at these examples:

1.Pizza, which most people love, is not very healthy.

2.The people whose names are on the list will go to camp.

But be aware, If this is a story and the rats are known you need to use comma.

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