For example, when you use the verb need in a sentence, you can use an infinitive clause after need, as in the first sentence below, but you will never use a finite clause as in the second sentence.

There is no rule as such. All we can say is that the to-infinitive clause is used after certain abstract nouns to show what action they relate to. So we can say: ...

There is no rule as such. All we can say is that the to-infinitive clause is used after certain abstract nouns to show what action they relate to. So we can say: ...