Why is the element one in words such as alone and only not pronounced like the numeral one?
Both the indefinite article an (a before consonant) and the numeral one are from Old English ān—which has survived in Scotland as ane, used both as indefinite article and as numeral.
This Old-English word ān meant a/an, one, and its plural āne meant some, certain. It is of Germanic origin and related to Dutch een and German ein. Those Germanic words are cognate with Latin unus/una, which is the origin of French un, feminine une. Those Dutch, German and French words mean both a/an and one: unlike English, those, and other, European languages, have retained the identity between the indefinite article and the numeral.
♦ When used as an indefinite article, ān was without emphasis and became short in quantity, whence the Modern English forms an, and a before consonant [cf. note 1]. The modern indefinite article an is therefore the older, fuller form of a, -n having disappeared before consonants.
♦ But when used as a numeral, ān retained its emphasis and quantity, and developed regularly into one, pronounced as in alone and only.
The initial w sound of one developed afterwards, before the 15th century, and was occasionally represented in the spelling [cf. note 2]. It was not accepted into standard English until the late 17th century.
This initial w sound also appeared in ones (from Old English ānes, genitive of ān) which is now spelt once; this spelling change occurred in the 16th century in order to retain the unvoiced sound of the final consonant.
But this pronunciation with a w sound did not affect the compounds alone, only and atone, which can therefore be compared to colloquial forms such as good ’un and young ’un.
The word alone is, literally, all one, meaning quite solitary. There are similar compounds in German with allein, in Dutch with alleen, and in Danish with alene.
Etymologically speaking therefore, all alone is a pleonasm.
The word lone is a shorter form of alone (in the same way, alive has been shortened to live in the sense being in life, as in a live animal). The adjective lonely is in turn composed of lone and of the suffix -ly [cf. note 3].
The word only, from Old English ānlic, was originally an adjective meaning sole of his, her, etc., like, i.e. sole of his, her, etc. kind [cf. note 3].
The original pronunciation of one is also found in the verb atone, from the adverb at one, meaning in a position of unity.
A PARTICULAR CASE: NONCE
The original pronunciation of one has also been preserved in nonce, used in for the nonce and in a nonce word. Here, nonce is an alteration by wrong division [cf. note 1] of early Middle English anes appearing in þen anes, from:
– þen, oblique form of þe (the),
– ane, one,
– the suffix -s (forming adverbs such as needs, meaning of necessity).
Note 1: In some cases, the final -n of an was understood as being the initial letter of the following noun. For example, the initial n- of newt is from a wrong division of an ewt (the noun ewt is from Old English efeta, which is also the origin of eft, so that newt and eft are etymological twins).
In some other cases, the opposite phenomenon took place: the initial n- of a noun was understood as being the final -n of the preceding article – hence, for example, apron, from Middle English naperon, of French origin (cf. Modern French un napperon, a tablemat, a doily).
Note 2: The initial w sound originated in western England and in Wales. In southwestern England, it appeared in other words beginning with the long o sound such as oats, pronounced there as wuts. But only in one and once did this pronunciation gain widespread usage. The spelling of one is won in the translation of the New Testament (1526) by William Tyndale (circa 1494-1536), who was from Gloucestershire, in southwestern England; the following, for example, is from the gospel of Matthew, 25:40:
Ī [= In] as moche as ye have done it vnto won of the leest of these my brethren: ye have done it to me.
Note 3: The Old-English suffix -lic is the origin of the modern suffix -ly, which forms adjectives meaning having the qualities of, as in lonely, brotherly. This suffix is related to alike and to the preposition like meaning having the same characteristics or qualities as, similar to, as in they were like brothers.