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I'm trying to analyse the following sentence.

To understand the importance of this event you should know all the facts.

It seems to me that this sentence is complex, and «To understand importance of this event» is dependent adverbial clause. But then, as my grammar book says, it would require a comma before «you», since the dependent adverbial clause comes before the independent clause.

So, please help to do the syntax analysis of this sentence and to understand whether the comma is needed.

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up vote 3 down vote accepted

You can divide this sentence into two separate clauses.

The dependent clause, which provides the reason, is:

To understand the importance of this event

while the independent (statement) clause is:

you should know all the facts

Identifying these two separate clauses, it is then clear that a comma should be introduced between them, by standard rules.

Hence it's most correct to write:

To understand the importance of this event, you should know all the facts.

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Yet the sentence is not so complex that omitting the comma renders it unparseable to the eye or ear. Read the exaample sentence aloud with and without the pause indicated by the comma. To my ear no pause and thus no comma is needed. – John Satta Jan 15 '11 at 21:31
@John: I would say it with a very short pause, as I think most speakers of British English would. In any case the "vocative comma" is only one of many commas! A comma is always needed to separate two clauses like this, if you're being strict. – Noldorin Jan 15 '11 at 21:33
@Noldorin: Fair point. If the sentence were reversed, would you still strictly need a comma between "...the facts" and "to understand..."? – John Satta Jan 15 '11 at 22:11
@John Satta: Indeed, if the two clauses were reversed, you would not need a comma. In fact I'd argue a comma would be wrong there. (What I might do is use "in order to understand" instead of just "to understand" for clarity.) – Noldorin Jan 15 '11 at 22:15
So the rules are slightly more subtle - I think it's safe to say a comma is needed when a dependant clause is followed by an independent one. – Noldorin Jan 15 '11 at 22:16

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