In English more easily than in most languages a word can be converted, i.e. made into another part of speech.
This usually takes place without any modification whatever, except, of course, the necessary change of inflection.Thus the noun eye is converted into a verb by merely giving it verbal inflection: 'They eyed the prisoners with curiosity.'
As adjectives are now always uninflected, the conversion of nouns, adverbs, phrases, and sentences into adjectives is very easy.
On the other hand, the conversion of adjectives into nouns is more difficult and irregular.
In old English, adjectives, converted into nouns, often retained their old adjective form. In many cases this old usage survived even after the adjective endings had disappeared; in other cases the loss of the adjective endings brought about new forms of expression. The breakdown of the adjective inflection at the close of the Middle English period forced the English people, who are fond of short-cuts in language, to do something contrary to their nature to go a roundabout way to express themselves. If we now say the good it can only mean that which is good, but in older English, according to the form of article, it could mean the good man, the good woman, the good thing. We now regularly use man, woman, and thing here, but there are numerous individual survivals of the older use of the simple adjective where the situation of itself without the help of the form of article or adjective makes the thought clear. Of persons: the deceased; the dear departed; my intended; the accused; the condemned; a lover clasping his fairest; my dearest (in direct address); etc. In a few cases a modern genitive form has been created: the Almighty's strong arm; her betrothed's sudden death; etc. A large number have a genitive singular in 's and a plural in -s, since they have become established as regular nouns: a savage, genitive a savage's, plural sav ages.
These nouns without an ending in the plural have been preserved because in the competition between the old and the new plural in older English they became differentiated in meaning. They acquired collective force: 'the poor of our city,' but 'the two poor men entering the gate'; 'the state of the heathen and their hope of salvation,' but 'Smith and Jones are regular heathens.' On account of the lack of a plural ending the old uninflected plural, however, is usually ambiguous, so that we often cannot use it at all. We may say 'the poor of the South,' but we must say 'the blacks (or the black people) of the South,' for the black now suggests a singular idea since it is sometimes used in the singular, thus now being felt as a noun: "Fetch a light," she said to the black who opened for us' (S. Weir Mitchell, Hugh Wynne, Ch. XXVII). We say also 'the whites of the South.' The old form is thus in quite limited use. A pastor might say to his congregation 'I urge old and young,' but he could not say 'I desire to meet after our service the young.' He would say the young people. But we say 'a picture of a willow-wren feeding its young' (or young ones). In a broad sense the young is used also of human beings: 'Men rode up every minute and joined us, while from each village the adventurous young ran afoot to enter our ranks' (T. E. Lawrence, Revolt in the Desert, p. 303). Since the names of some peoples have been made from adjectives, as the English, the French, the old uninflected adjective plural has become productive here, and is now used with many names of peoples: the Swiss (in older English Swisses), Portuguese (in older English Portugueses), Japanese, Chinese, etc. We sometimes use the same form for the singular just as we use 'the deceased' for the singular, but we avoid these singulars since we feel these forms as plurals and prefer to say 'a Portuguese gentleman, lady,' etc.
In some cases we make nouns out of the substantive form, i.e. the one-form: the Crucified One; the Evil One. 'He is a queer one. My dear ones; our little ones; my loved ones; the great ones of earth, etc.
In a few cases nouns made from adjectives may drop the article as in older English: 'My good lady made me proud as proud can be' (Richardson, Pamela, 1II, 241). 'Eleven years old does this sort of thing very easily' (De Morgan, Joseph Vance, Ch. XV). 'Sweet Seventeen is given to daydreams.' 'Slow and steady wins the race. For 'tis the eternal law That first in beauty should be first in might' (Keats, Hyperion, II, 228). First come, first served.' 'First come, first in.' In plain prose an article is usually placed before the noun: 'He is strong for an eleven-year-old.' 'I was the first one served.' 'We were the first ones served.'
Nouns made from adjectives often denote lifeless things, usually with a meaning more or less general or indefinite. They are usually preceded by the definite article or some other limiting adjective: the present (the present time); the beautiful; the sublime. You ask the impossible.' 'He did his best.' As such forms, though now employed as nouns, were originally adjectives, they still are often, like adjectives, modified by adverbs: the genuinely lovable; the relatively unknown; etc. There are still many neuter nouns made from adjectives, but in older English the tendency to use them was stronger than today.
A number of these nouns have since been replaced by other words: 'Let me enjoy my private' (Shakespeare, Twelfth Night, III, II, 99), now privacy. Whereat a sudden pale (now paleness) Usurps her cheeks' (id., Venus and Adonis, 589).
While the neuter nouns made from adjectives now usually have the definite article or some other limiting adjective before them, we still not infrequently find the older articleless form, especially in the case of two adjectives connected by and: 'I can spy already a strain of hard and headstrong in him' (Tennyson). 'That is good, but there is better to follow.' 'There is worse ahead.'
The modified or unmodified form has become fixed in many set expressions: in the dark; after dark; through thick and thin; from grave to gay; to keep to the right; to go to the bad; to go from bad to worse; to make short of long; the long and the short of it; before long. After frequent interchange of foul and fair' (Tennyson, Enoch Arden, 529). "The police came up to see fair between both sides' (London Daily News, Mar. 11, 1891). 'That's no fair' (common in the language of children).
A large number of neuters have become concrete nouns: Ger man; Luther's German; the German of the present time; my German; a daily (paper), pl. dailies; a weekly, pl. weeklies; the white of an egg, the whites of eggs. 'They sent him a wireless.' 'He at last got in one with his left' (left hand). "The combats between the moderates and the extreme left' ( more democratic section of European legislative chamber). 'What is the good of lying?' 'It is no good trying to conceal it,' but the plural goods has a much more concrete meaning. A large number are employed only in the plural: greens, woolens, tights, necessaries, movables, valuables, the Rockies, etc.
Most of the adjectives used as nouns in the examples given above are descriptive adjectives, but also some limiting adjectives are used as nouns: 'He has lost his all.' 'He and his are all well.' 'I wrote you the details in my last' (= last letter). 'He was successful from the first' (= the beginning).
Proper adjectives are often limiting adjectives. They can, of course, be used also as nouns: a German; a German's; the Germans; etc.