Infinitive as NounVerbal Noun in -ing
To swim was difficult.Swimming was difficult.
My business is to make shoes.My business is making shoes.
To see is to believe.Seeing is believing.

Nouns in -ing are sometimes called infinitives or gerunds.

351. A noun in -ing may be used as an adjective, or as the adjective element in a compound noun ([§ 64]).

Note. Other examples are:—a working day, an ironing board, drinking water, smelling salts, marching orders, a walking tour, a swimming race, a vaulting pole. In such cases it makes little difference whether the two nouns are taken together as a compound, or whether the first is regarded as an adjective modifying the second. The difference between this use and that of the participle is perfectly clear. A “sleeping dog” is a dog that sleeps; a “sleeping car” is a car for sleeping. Sometimes, indeed, either explanation is possible. Thus, a “hoisting engine” may be understood either as an “engine that hoists,” or as an “engine for hoisting.” But it is better to class these exceptions with the nouns in -ing.

352. When a verbal noun in -ing is preceded by an article or any other adjective, it cannot take an object.

Observe that, in each instance, the object (song-birds, ship, maps, confectionery, wells) is replaced by a prepositional phrase when an article or other adjective precedes the verbal noun.

Note. In such expressions as “I went a-fishing,” a is a shortened form of the preposition on, and fishing is a verbal noun used as its object. When a is omitted we have “I went fishing,” “The house is building,” and the like, in which the word in -ing seems to be a participle, but is really the object of the omitted a (= on).

353. The possessive case of a noun or pronoun may be used to limit a verbal noun in -ing.