Learn English – Why is “fridge” spelt with a ‘d’ but “refrigeration” spelt without one

Interesting question :) Reading up on this has been a pretty crazy experience. My conclusions are the following:

• The reason for the change from fulfil to fulfill is attributed to inflection, with the spelling modified to suit the function of the word.
• The reason why it isn't spelt fullfill is similar to why parallel only retains the single 'l' at the end—to curb the apparent fugliness of an '-llell' cluster in words such as unparalleled etc.
• There is also a question of consistency involved.

But, unfortunately, another rule in the AE transition was to preserve the "base" word as much as possible, which is why skill/_skilful in BE became skill/skillful in AE. However, this introduced a cluster of letters in words such as skillfully. To put it mildly, the result of exercise was and remains consistently inconsistent.

A related question on the difference in the AE/BE spellings of propelling covers similar territory. It also points to a WP page detailing the differences between American and British spelling which includes the following section on the madness of doubled consonants:

• The word parallel keeps a single -l- in British English, as in American English (paralleling, unparalleled), to avoid the unappealing cluster -llell-.
• Words with two vowels before a final l are also spelled with -ll- in British English before a suffix when the first vowel either acts as a consonant (equalling and initialled; in the United States, equaling or initialed), or belongs to a separate syllable (British fu•el•ling and di•alled; American fu•el•ing and di•aled).
  • British woollen is a further exception due to the double vowel (American: woolen). Also, wooly is accepted in American English, though woolly prevails in both systems.

There's also a note on -ful and -fil:

In both American and British usages, words normally spelled -ll usually drop the second l when used as prefixes or suffixes, for example full→useful, handful; all→almighty, altogether; well→welfare, welcome; chill→chilblain. The British fulfil and American fulfill are never fullfill or fullfil. Johnson wavered on this issue. His dictionary of 1755 lemmatises distil and instill, downhil and uphill.

I think part of the premise of this question is incorrect: build did not gain a "u" between Middle English and Modern English. Modern English spelling is standardized, so we can talk about "the" spelling of most specific words. But in Middle English, there are usually multiple attested spellings for the same word; this is partly due to overall lack of standardization (some writers might even spell the same word several different ways on a single page), and partly due to the existence of conflicting standards (Middle English is a cover term that encompasses many different regional varieties over a pretty wide range of time, traditionally from the Norman Conquest of 1066 up until the Great Vowel Shift around the 1500s).

Dictionaries, such as Wiktionary, need to choose one written form to use as the headword (in this case, "bilden"), but this doesn't mean that no other forms existed during this time period. According to the Oxford English Dictionary (OED), forms of this word spelled with "ui" or "uy" actually date back to Middle English, but their origin is unclear:

Etymology: Middle English bulden(ü), bylden, bilden < Old English *byldan to build (recorded only in past participle gebyld), < bold a dwelling. Hence the two fundamental senses are ‘to construct a dwelling’ and ‘to take up one's abode, dwell’. The normal modern spelling of the word would be bild (as it is actually pronounced); the origin of the spelling bui- (buy- in Caxton), and its retention to modern times, are difficult of explanation.

My guess is that it has something to do with phonetic rounding, as the letter "u" is generally associated with rounded vowels. This could be due to the preceding rounded consonant /b/, the historical rounding of the root vowel (Old English y was the rounded counterpart of i) or some combination of both. I'm not at all sure of this, because as the OED says, the regular development of Old English y is complete loss of rounding and merger with i, as in words like kiss v. (from Old English cyssan v.) or lice n. (from Old English lȳs n.).

However, modern standard English vocabulary does also show some influence, in both pronunciation and spelling, from dialects where y developed differently. (I'll describe this in more detail later by quoting the etymologies of specific words.) According to the link that Ricky found (Notes on Etymology by Walter W. Skeat 1901), the digraph "ui/uy" was used in Southern Middle English to represent the sound that developed from older /yː/. Skeat says this is the reason for the spelling of build (which apparently had a long vowel at this point due to homorganic lengthening before the consonant cluster -ld). To support this, he lists the words bruise, from Old English brȳsan, and buy, from a form of Old English bycgan.

I was not able to find any other examples of "ui/uy" being used this way in modern spelling. However, the OED does list muys, muyse as Middle-English spellings of the word mice (from Old English mȳs). For comparison, ice (from Old English īs, without historical rounding) does not have any spellings listed with "ui" or "uy."

Buy seems to be particularly relevant because, like build, it is pronounced with an unrounded vowel in modern English. The OED gives the following information about the etymology:

Old English bycg(e)an, bohte, geboht, corresponding to Old Saxon buggjan, *bohta, giboht, Gothic bugjan, bauhta, bauhts; of unknown origin, not found outside Germanic, and not to be connected, so far as can be seen, with the stem bug- bow n.1 The inflection was imperative byge, bycgað; indicative present bycge, bygest, bygeþ, plural bycgað; subjunctive present bycge, bycgen; whence Middle English s.w. buye, buggeþ; bugge, buyest, buyeþ, buggeþ; bugge, -en; levelled before 1500 to buy- all through, whence the modern spelling. The forms in begge, bey- were Kentish; bigge, bie, by, midland and northern; in the latter the levelling to bie, by, took place as early as 1300.

The forms with the long consonant “cg” seem to have been eliminated through leveling. Another verb that seems to have developed in a similar way is licgan "to lie (down)”: both of these words have the diphthong /aɪ/ in modern pronunciation, which generally develops from Middle English /iː/. But the different spellings suggest that buy may have been pronounced with another vowel at some point, or in some dialects. It's not clear to me from this entry if southwest buy- is the ancestor of the modern pronunciation as well as the spelling (I'm not sure how that development would work in terms of sound changes) or if the modern pronunciation simply comes from midland and northern bie, by.

Google-research suggests that aside from build, buy, and related words, there are no other words where "bui/buy" is pronounced like "bi." (This spelling pattern is discussed in A Survey of English Spelling, by Edward Carney, and Dictionary of the British English Spelling System, by Greg Brooks; both of these sources use the synchronic analysis that "bu" acts as a symbol for /b/.) There are some other words that are superficially similar (buoy, guild, guy, biscuit, conduit, Kuiper) but in general they have different explanations for their spelling, so they are not very useful for explaining build.

However, it might be useful to compare it to busy, for which the OED says:

The original stem vowel ĭ is shown by Old English bisig; the form bysig (when not simply an inverted spelling with y for i in areas where Old English y had been unrounded) probably shows late West Saxon rounding of the stem vowel as a result of the influence of the preceding labial consonant (compare A. Campbell Old Eng. Gram. (1959) §318); Middle English (and modern standard English) busy continues this form (although in the case of the modern standard form with the pronunciation of the unrounded variant). Pronunciation with /ɪ/ is regularly indicated for forms spelt with -u- by orthoepists from the mid 16th cent. onwards (see E. J. Dobson Eng. Pronunc. 1500–1700 (ed. 2, 1968) II. §82).

Forms such as Middle English and early modern English besy (see γ. >forms) reflect Open Syllable Lengthening of short ĭ to long close ē.

Another relevant word is bury, from Old English byrgan. According to the Online Etymology Dictionary:

Under normal circumstances [Old English -y-] transformed into Modern English -i- (as in bridge, kiss, listen, sister), but in bury and a few other words (as in merry, knell) it retained a Kentish change to "e" that took place in the late Old English period. In the West Midlands, meanwhile, the Old English -y- sound persisted, slightly modified over time, giving the standard modern pronunciation of blush, much, church.

It looks like the modern pronunciation of bury comes from dialects like Kentish, while the spelling comes from dialects like those in the West Midlands.

Build, buy, busy, and bury all have a "b" before the vowel: this is the "labial consonant" mentioned by the OED. The idea that it might have influenced the development of the following vowel in words like these is plausible, since there is strong evidence that it caused the following vowel to retain rounding in words like bull. However, it seems that in some dialects, there were words that retained rounding from Old English y even when it was not directly preceded by a labial consonant, such as bruise, blush, and church.