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Which one is correct:

To perform a task is necessary to do some things.


To perform a task it is necessary to do some things.

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up vote 5 down vote accepted

You need it:

To perform a task, it is necessary to do some things.

The reason is that, in your sentence without it, the subject to do some things would come after the finite verb is: because it is generally not possible to have no subject before the verb, you need a dummy subject it to come before the verb, at least in non-inverted word order.

I think most people would add the comma here, though I'd not consider it absolutely necessary.

You even need it when used in a relative clause, according to style books:

The Hanseatic League, which it is necessary to have on our side, still refuses to boycott Spanish traders.

The reason is that you couldn't replace it with the subject of the subordinate clause. Which refers to the Hanseatic League (the antecedent), and we should then be able to replace which with its antecedent:

? The Hanseatic League is necessary to have on our side ...

But this sounds slightly off or different. It is not the Hanseatic League that is necessary in order to have x on our side: it is having the H. L. on our side as a whole that is necessary for some unmentioned purpose. (If this were possible, it would be a case of raising, as with it seems she has left us => she seems to have left us, where there is no difference in meaning.) We'd still need a dummy subject here:

It is necessary to have the Hanseatic League on our side ...

Note that it would sound more natural to say we need the H. L. on our side.

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Both are correct but they have different meanings. In addition, I would not use the first example.

Suppose that you have to click a button when you want to print out a document (in a computer program). Then I could say:

  1. To print out a document, it is necessary to click a button.
  2. Clicking a button is necessary to print out a document.
  3. To click a button is necessary to print out a document.

Note the difference in the order.

I prefer to use a comma in 1, but some people omit it.

I would not use 3 because it is hard to understand for me. This is because when I see “To click a button” at the beginning of the sentence, I expect a sentence like the first one and therefore expect to see a subject later, but actually “To click a button” is the subject. However, I would not say 3 is incorrect.

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I do not understand the silent downvoting here. Some people seem to assume that “to perform a task” is a goal and “to do some things” is a means, but if it is the opposite, the first example in the question is correct (although it is hard to understand). And from the vague example in the question, I cannot tell which is a goal and which is a means. – Tsuyoshi Ito Apr 27 '11 at 12:45
I would not use 3 either and so, without a good reason, I suspect it may not be correct – Henry Apr 27 '11 at 13:17
I agree that 3 is bad style at best: two constituents with to-infinitives is hard to parse. – Cerberus Apr 27 '11 at 13:25
@Cerberus: I agree that 3 is bad style. As Colin Fine writes in a comment on JSBangs’s answer, it is a garden-path sentence (I had been forgetting this phrase when I wrote this answer), at least for me. – Tsuyoshi Ito Apr 27 '11 at 13:30

You should use the second one, but it's a bit unclear (in written form). Insert a comma:

To perform a task, it is necessary to do some things.

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Your second option is correct:

To perform a task it is necessary to do some things.

The reason for this is that the verb is needs a subject. Verbs in English always have to have an explicit subject, with a handful of exceptions (the imperative being the most common). Therefore, whenever we have a statement with no logical subject, we have to add the word it (or sometimes there) in order to act as a placeholder subject. For example:

It is raining.

The words it doesn't refer to anything in particular here, but it's required by the grammar of English to supply a subject.

In your sentence, you might be tempted to analyze the infinitive to perform a task as the subject. This is incorrect, however. The infinitive here is actually an adjunct, with an understood prepositional in order preceding it. You can demonstrate this by moving the infinitive to the end of the sentence:

It is necessary to do some things in order to perform a task.

Here's it's clear that to perform a task is an adjunct, and that the verb is requires the dummy subject it. The same thing applies, though somewhat less obviously, when the infinitive is moved to the beginning of the sentence.

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As Tsuyoshi Ito says, the reading with "to perform a task" is grammatical, but has a different meaning. This makes it something of a garden-path sentence, so it is unlikely to occur, but it could. – Colin Fine Apr 27 '11 at 13:03

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