What you’re dealing with here is the difference between:
adjective + preposition + gerund
adjective + infinitive.
They’re two different kinds of constructions.
Let’s do adjective + preposition + gerund first.
Some adjectives are frequently followed by prepositions. The only form of a verb that can follow a preposition is a gerund, because a gerund or gerund phrase basically functions as a noun or noun phrase. So you get:
Fertilizer is important to raising healthy plants.
“Fertilizer” is the subject, “to” is a preposition, and “raising” is a gerund.
Research is essential to making the right purchase.
“Research” is the subject, “to” is a preposition, and “making” is the gerund.
His voice was hoarse from screaming.
“His voice” is the subject, “from” is a preposition, and “screaming” is a gerund.
Now, the adjective + infinitive.
These sentences are kind of complicated. There’s usually an it somewhere as either a dummy subject or a dummy object. The it is a place holder for the infinitive phrase, which is the real subject or object.
Look at these old-fashioned sounding, but correct, sentences. In them, the subject is the infinitive phrase.
[color=darkred]To finish now would be impossible.
[color=darkred]To make a good impression is extremely important.
It is possible – and very typical – to move the infinitive phrase to the predicate and put it in the subject position where the infinitive came from.
[color=darkred]It would be impossible [color=darkred]to finish now.
[color=darkred]It is extremely important [color=darkred]to make a good impression.