- It's hard for you to make such a decision.
- It's very kind of you to see me off.
Either of the above sentences works well. Here arises a question about how to use the prepositional phrase preceding the infinitive? For example,
- It is wise of him ( or "for him" ?) to say so.
Given a construction like
It is [adjective] [of/for] him to [verb phrase]
Here are some general rules:
We can use of him whenever the adjective could reasonably be applied to him. This can often be tricky, so here are a few examples:
It's cruel of you to leave me here in the cold. ✔ (you are cruel)
It's cool of him to give so much money away. ✔ (he is cool)
It's surprising of her to do that. X (she is surprising? dosn't really work)
It's unfair of you to give her more than me. ✔ (you are unfair)
It's hard of you to make such a decision. X (you are hard?)
We can use for him in general when of him doesn't apply, but also when the adjective applies to the person in question specifically and would not apply to just anyone in that person's shoes. I'll explain this more below, but first, here are some examples of the first and simplest case:
It was huge for him to score this win. ✔ (he is not huge)
It was dangerous for him to climb up there. ✔ (he is not dangerous)
The second case is somewhat more complex. Sometimes we use for you to emphasize the circumstances of the person in question. Immagine the following two scenarios:
A) A group of gang members walks up to John. They ask him "Do you support our gang?" John says, "Yes, I love your gang!" They let him go without bothering him.
B) A group of gang members walks up to John. They ask him "Do you support our gang?" John says, "No, I don't support gangs." They rob John and then let him go.
In scenario (A), it was wise for John to express support for the gang because of the particulars of his circumstance. In other words, because of the nature of John's specific circumstance, it was smart. In scenario (B), it was wise of John to say that he doesn't support gangs; in fact, it would be wise of everyone to do so regardless of their circumstance. In scenario (A), you can use either for or of if you want to describe John as wise: the adjective can apply to John, and the wiseness may be due to his specific circumstances. I would use for if you want to emphasize the circumstances, of if you want to emphasize the principle of the action.
In short: In your example (3), you can use either of him or for him, depending on if you want to emphasize his specific situation or the general wise-ness of the action.
Unfortunately, English prepositions are some of the hardest and least consistent parts of the language.
