The Penguin Dictionary of American English Usage and Style : A ...

infinitive 185
and in the first person singular (except
for I am), I think or I see.
Usually an infinitive is indicated by to:
“He wanted to know.”/ “She needs to
leave.”
Infinitives without to regularly follow
some verbs, such as can, let, may, might,
must, and should (“The man can run” /
“You should try”), and appear in certain
constructions (“A crowd watched them
fight” / “I will quit rather than move”).
To plus infinitive may act as a noun,
either in the subject (“To build is a noble
art”) or in the predicate (“Maria loves to
sing”). It may also act as an adjective
(“John has an ambition to fly”) or as an
adverb (“They came to help”).
One to normally suffices for multiple
infinitives of similar construction: “I
want to finish my work and go home”—
the second to is understood. But a subsequent
to may be desirable for emphasis:
“I come to bury Caesar, not to praise
him.”
The use of to to indicate the infinitive
(“I want to buy a pig”) should not be
confused with any other use of to, e.g.,
to indicate direction or purpose (“I’m
going to town to buy a pig”). See TO.
See also Verbal, 3; Verbs, 1.
2. Gerund versus infinitive
When to use the gerund and when to
use the infinitive puzzles some writers
and speakers. It is the difference between
seeing and to see, between laughing and
to laugh. The first is the -ing form of a
verb when it serves as a noun. The second
is the basic form of a verb, preceded
by to. For instance, a broadcaster said:
Not all dry cleaners share Robin
B——’s enthusiasm to protect the environment.
A more idiomatic phrasing would be
“enthusiasm for protecting the environment.”
Some other nouns would go with
the infinitive: e.g., “desire to protect” /
“wish to protect.” Still other nouns
would go with the gerund: e.g., “concern
for protecting” / “idea of protecting.”
An infinitive is needed in this sentence,
by a news agency:
HELSINKI—President Boris N.
Yeltsin of Russia yesterday acknowledged
his failure in winning President
Clinton’s promise that no former Soviet
republic will ever be allowed to
join NATO.
“In winning” should be to win. A correct
example of the former: “She is encouraged
by her success in winning the
tournament.”
There are no rules pointing to one
form or the other. It is a matter of idiom
and knowing how each verb is used. Dictionary
examples can be instructive.
See also Gerund, 3.
3. Perfect infinitive
A form that borrows the term infinitive
but should not be confused with the
form defined in 1 is the perfect infinitive.
Examples are to have gone, to have
made, and to have sung. Using to have
and a past participle, it normally expresses
action that is, was, or will be
completed before another action or
event indicated in the same sentence.
“I’m happy to have won your confidence.”
/ “The office seemed to have
been ransacked.” / “They plan to have
built the house by the end of the year.”
Sometimes the perfect infinitive is unneeded,
as in this sentence from a novel:
“He would have liked to have hugged
his father.” Probably at that moment he
would have liked to hug his father.
Another example of a misuse of the
perfect infinitive: “I planned to have
moved.” It reverses the logical time sequence.
Make it “I had planned to
move”; the planning had to precede the
moving.
“He wants to be the first to have