The infinitive is used in various noun constructions such as subject, as predicate nominative, as nominative of exclamation, as appositive, as object of certain prepositions and as modifier.
An infinitive with or without a complement or modifiers may be used as the subject of a sentence, as a predicate nominative or as an appositive.
1. To descend was extremely difficult. [Subject.]
2. To secure a seat was impossible.
3. To sing well requires practice.
4. His delight was to travel. [Predicate nominative.]
5. The governor’s policy is to wait.
6. My wish is to see you immediately.
7. To decide was to act. [The first infinitive is the subject, and the second is a predicate nominative.]
8. Both alternatives, to advance and to retreat, seemed equally hazardous. [Apposition with the subject.]
9. My first plan, to tunnel under the wall, proved a failure.
10. He has but one aim in life, to succeed. [Apposition with the object.]
11. I have written with a definite purpose, to dissuade you.
12. I give you three choices — to buy, to lease or to build.
An infinitive in the predicate is often in apposition with the expletive subject it.
1. It was a pleasure to see him. [Instead of: To see him was a pleasure.]
2. It is easy to understand you.
3. It will be impossible to forget.
4. It proved very difficult to find evidence against him.
In this use….the infinitive, though grammatically in apposition with it, is really the subject of the thought.
The infinitive may be used as the object of the prepositions but, except, about.
1. There was nothing to do but walk (or to walk).
2. He will do anything except resign (or except to resign).
3. We are about to object. [An idiom expressing futurity.]
4. The train is about to start.
Note : Can BUT and cannot BUT are distinct idioms.
(1) In…..I can but thank you…..BUT is an adverb (= only). The sentence means…..I can only thank you - simply that and nothing more!
(2) In….I cannot but thank you….BUT is a preposition (= except). The idiom is shortened from “I cannot choose but thank you,"—that is, “I have no choice except to do so," or, in other words, “I cannot help it."
The infinitive after for (now a gross error) was once in good use such as…
1. What sweeter music can we bring
2. Than a carol for to sing. - Herrick.
The infinitive may be used as a nominative of exclamation.