-ing is a suffix used to make one of the inflected forms of English verbs. This verb form is used as a present participle, as a gerund, and sometimes as an independent noun or adjective. The suffix is also found in certain fossilized words like morning and ceiling, and in names such as Browning.
Etymology and pronunciation
The Modern English -ing ending, which is used to form both gerunds and present participles of verbs (i.e. in noun and adjective uses), derives from two different historical suffixes.
The gerund (noun) use comes from Middle English -ing, which is from Old English -ing, -ung (suffixes forming nouns from verbs). These in turn are from Proto-Germanic *-inga-, *-unga-, *-ingō, *-ungō, which Vittore Pisani derives from Proto-Indo-European *-enkw-. This use of English -ing is thus cognate with the -ing suffix of Dutch, West Frisian, and the North Germanic languages, and with German -ung.
The -ing of Modern English in its participial (adjectival) use comes from Middle English -inge, -ynge, alterations of earlier -inde, -ende, -and, from the Old English present participle ending -ende. This is from Proto-Germanic *-andz, from the Proto-Indo-European *-nt-. This use of English -ing is cognate with Dutch and German -end, Swedish -ande, -ende, Latin -ans, -ant-, Ancient Greek -ον (-on), and Sanskrit -ant. -inde, -ende, -and later merged with the noun and gerund suffix -ing , perhaps because the French speaking elites naturally confused them as there was only one French equivalent to both (compare the Modern French participial, noun and gerund suffix -ant). Its remnants, however, are still retained in a few verb-derived words such as friend, fiend, errand, thousand, bond, etc.
The standard pronunciation of the ending -ing in modern English is "as spelt", namely /ɪŋ/, with a velar nasal consonant (the typical ng sound also found in words like thing and bang); some dialects, e.g. in Northern England, have /ɪŋg/ instead. However many dialects use, at least some of the time and in some cases exclusively, an ordinary n sound instead (an alveolar nasal consonant), with the ending pronounced as /ɪn/ or /ən/. This may be denoted in eye dialect writing with the use of an apostrophe to represent the apparent "missing g"; for example runnin' in place of running. For more detail see g-dropping.
All English verbs (except for modals and other defective verbs which do not have gerunds or participles) make the inflected form in -ing regularly. Thus go makes going, read makes reading, fail makes failing, and so on. In certain cases there are spelling changes, such as doubling of consonants (as in sit → sitting) or omission of mute e (as in change → changing). For details of these rules, see English verbs.
The -ing form of a verb has both noun uses and adjectival (or adverbial) uses. In either case it may function as a non-finite verb (for example, by taking direct objects), or as a pure noun or adjective. When it behaves as a non-finite verb, it is called a gerund in the noun case, and a present participle in the adjectival or adverbial case. Uses as pure noun or adjective may be called deverbal uses.
The distinctions between these uses are explained in the following sections.
Distinction between gerunds and present participles
Gerunds and present participles are two types of non-finite verb; the difference is that gerunds are used to produce noun phrases, and participles to produce adjectival or adverbial phrases. This is illustrated in the following examples:
Confusion is most likely to arise when the -ing word follows a verb, in which case it may be a predicate adjective and hence a participle, or a direct object (or predicate nominative) and hence a gerund. There are certain transformations that can help distinguish these two cases. In the table that follows, the transformations produce grammatical sentences with similar meanings when applied to sentences with gerunds (since the transformations are based on the assumption that the phrase with the -ing word is a noun phrase). When applied to sentences with participles, they produce ungrammatical sentences or sentences with completely different meanings. (These cases are marked with asterisks.)
For more details of the usage of English gerunds and present participles, see Uses of non-finite verbs in English.
Distinction between verbal and deverbal uses
When used as a gerund or present participle, the -ing form is a non-finite verb, which behaves like a (finite) verb in that it forms a verb phrase, taking typical verb dependents and modifiers such as objects and adverbs. That verb phrase is then used within a larger sentence, with the function of an adjective or adverb (in the case of the participle) or with the function of a noun (in the case of the gerund).
However the same verb-derived -ing forms are also sometimes used as pure nouns or adjectives. In this case the word does not form a verb phrase; any modifiers it takes will be of a grammatical kind which is appropriate to a noun or adjective respectively.
When used as a pure noun or adjective (i.e. having lost its grammatical verbal character), the -ing form may be called a deverbal noun or deverbal adjective. Terminology varies, however; it may also be called a verbal noun or adjective (on the grounds that it is derived from a verb). In other cases the latter terms may be applied additionally, or exclusively, to gerunds and participles, as well as other non-finite verb forms such as infinitives.
In some situations, the distinction between gerund/participle uses and deverbal uses may be lost, particularly when the -ing word appears on its own. For example, in "I like swimming", it is not clear whether swimming is intended as a gerund (as it would be in "I like swimming fast"), or as a pure noun (as in "I like competitive swimming"). Note that there may be a distinction in meaning between the two interpretations: as a gerund, it means that the speaker likes to swim, while as a pure noun it does not specify in what way the speaker enjoys the activity (as a competitor, spectator, etc.)
The -ing form used as a pure noun usually denotes the action encoded by the verb (either in general or in a particular instance), as in the above examples. However it sometimes comes to take on other meanings, such as a physical object or system of objects: building, fencing, piping, etc.
For more information on the uses of non-finite verbs and verbal nouns, see Uses of non-finite verbs in English.
-ing words in other languages
English words constructed from verbs with the ending -ing are sometimes borrowed into other languages. In some cases they become pseudo-anglicisms, taking on new meanings or uses which are not found in English. For instance:
Some Germanic languages (including Dutch, Danish, Swedish, Norwegian, and Icelandic) have a native -ing suffix, used mainly to form verbal action nouns, though generally not as productively as in English. For details, see the Wiktionary entry for -ing.
Other meanings of the suffix
The suffix -ing also has other uses in English, although these are less common. It may be used to form derivative nouns (originally masculine) with the sense "son of" or "belonging to", used as patronymics or diminutives. Examples of this use include surnames like Browning, Channing and Ewing, and common nouns like bunting, shilling, and farthing. The suffix can also mean "having a specified quality", as used in sweeting, whiting, and gelding.
For further details see the Wiktionary entry for -ing.