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Usually, we use the structure "for...to..." to indicate an infinitive with its own subject.

Anna will be happy for the children to help you. (The children will help you.)

My idea was for her to learn Russian.

It's important for the meeting to start tomorrow.

However, sometimes we use the preposition 'of' instead.

It's very kind of you to help.

It's silly of her not to accept him.

I wonder when we should use the preposition 'for' and when we should use 'of?'

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  • A long time ago, I seemed to know the answer. It has something to do with the role of the noun after the pronoun plays. For example, when we say “it’s kind of you to help me,” we are saying we think you are kind, but when we say “it’s important for you to do homework,” we are saying you need to do homework. The former is describing our opinion and the latter is providing a subject for the Infinitive. I vaguely remember it’s along these lines but cannot find any reference on this subject any more. Anyone who knows please chip in. – Tom Bennett Apr 29 at 19:42
  • It's possible some ambitious English teacher told you about some supposedly overarching rule regarding the use of "for" and "of", but I guarantee you for every example that fits the rule I can find an exception. – Andrew Apr 30 at 5:44
  • No, it was from a book. I lost the book and forgot the title. So you are saying it’s completely idiomatic? No rules? – Tom Bennett Apr 30 at 10:01
  • It's not completely idiomatic. "For" and "of" have different underlying purposes that map to similar particles in other languages. However I do think there are many exceptions to which words they pair with, so many that you can't really trust any overarching "rule". – Andrew Apr 30 at 18:21
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In English, preposition use is often about idiom not grammar. It's simply more natural to say "important for" or "important to" rather than "important of". Similarly, "It's kind of you," is generally more idiomatic than, "It's kind for you".

Once you get past the basic grammar, it's a good idea to memorize English phrasal verbs as well as common verb-preposition collocations. Simply memorizing the verb itself isn't enough. In the same way memorize common patterns with certain words, for example:

"happy" + infinitive: I was happy to hear that she got a promotion.

"happy" + for [someone/something]: I was happy for her that she got a promotion.

That being said, I think some of your examples are of different sentence structures. "For" can introduce a relative clause, as in your third example, which can be rephrased:

It is important that the meeting start tomorrow.

Be aware that words often pair with several different idioms, but with slightly different nuance.

It's silly of her not to accept his offer

implies that she is being silly, for the given reason. Meanwhile

It's silly for her not to accept his offer

implies that the given action or reason is silly. This may seem like a trivial difference, but in some circumstances it might be offensive to call someone silly, but less offensive to suggest that they are making silly choices.

You can apply this to other terms similar to "silly". Imagine (at some time in the past) a royal counselor speaking to the king or queen of England:

Your Majesty, I believe that it is unwise for you to provoke war with France at this time.

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  • Can you add some reference to the last part of your answer? I find the difference important and interesting. – Tom Bennett Apr 30 at 2:35
  • @TomBennett I'm not sure I can -- I wouldn't even know where to start looking. I suggest you keep my definition in mind and, if you come across it in the future, see if what I say matches with what other writers intend. – Andrew Apr 30 at 22:58
  • Many thanks! This is very helpful. – Tom Bennett May 1 at 0:50

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