Do I have to use the comma before the WHEN CLAUSE in the following sentence?
"We were swimming in the lake, when suddenly a crocodile attacked us."


It's not a matter of necessity; it's a matter of whether you wish to conform to the style recommended in your manual of style. I use the Chicago Manual of Style, which recommends that dependent clauses that follow an independent clause be set off by a comma if and only if the dependent clause is non-restrictive, i.e, if it's informational only instead of definitional.

Your example seems non-restrictive: you've merely added an interesting story to your swimming holiday. Here's a restrictive example:

Do you remember that swimming holiday when a crocodile suddenly attacked us?

Now the clause distinguishes that time when you went swimming from all the other, less eventful times. So no comma.

Punctuation is a matter of style, not a matter of life and death, like swimming in a crocodile-infested lake.

  • 1
    Similar to your "distinguishing that swimming trip from others (and hence no comma)" point - if the OP's question is in response to the question "what were you doing when the crocodile attacked you?", then I think the comma should be omitted. – Rupe Nov 12 '15 at 9:52
  • @deadrat Your explantion is very clear. So the point is to separate the non-defining (dependent) clause from the independent clause with a comma when it (the dependent clause) gives only additional information. Right? – mido mido Nov 12 '15 at 10:39
  • @midomido Right. – deadrat Nov 12 '15 at 11:06

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