From Longman dictionary I see that the word necessary can be used as follows:

It is necessary (for somebody) to do something.

Something is necessary for (doing) something.

However, I saw the sentence

Air is necessary to life;

in a book which I think should be

Air is necessary for life.

Which form is correct and why?

  • It seems that both are used. According to the link, in 67% of cases necessary for is used and in 9% of cases necessary to (not sure what cases have been considered). However, 67% also includes for X to do Y cases. – r.beigi Sep 16 '17 at 5:08
  • You must also keep context in mind. – Soha Farhin Pine Sep 16 '17 at 5:10
  • @SohaFarhinPine Interestingly, half of those "necessary to" examples are wrong, because the "to" is actually just for the infinitive verb that follows "necessary." – Ringo Sep 16 '17 at 5:44
  • 1
    @Ringo: Haha - food is necessary to live, as well as being ...necessary to life! – FumbleFingers Mar 22 '19 at 17:21

X is necessary for Y

This expresses a prerequisite. Here, you are saying that if you don't have Y, you can't have X.

X is necessary to Y

This can be interpreted as "Y believes X is necessary", but this may not match what you think is necessary or what actually is necessary.

In the case of life, air is objectively necessary (for some life anyway), so you can say to or for.

  • "... if you don't have Y, you can't have X" should be "... if you don't have X, you can't have Y". – r.beigi Mar 23 '19 at 5:49

To is used after necessary not only before infinitive but also for condition.

It is necessary to the happiness of man to live in comfort.

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