In 1988, research by J. C. Wells for the Longman Pronunciation Dictionary showed that only 27% of British English speakers pronounce the "t". Subsequently, 1993 research showed that only 22% of Americans pronounce the "t".
Whether things have changed in the subsequent thirty years I don't know. The major dictionaries include both variants, but continue to put the silent-'t' pronunciation first.
The silent 't' is considered traditional, and the 1965 edition of Fowler's Modern English Usage lamented the fact that the pronunciation with a "t" had become widespread. It saw this as part of the "speak-as-you-spell movement" in which native speakers were increasingly pronouncing things as they were written instead of in a more traditional way. Of course, in most cases this actually involves returning to an older pronunciation (and this is true of "often").
The Oxford English Dictionary records that the t-less pronunciation was avoided by careful speakers in the 17th century (despite having been used by Queen Elizabeth I) but subsequently became standard. Pronouncing the "t" was later regarded by some authorities as a "hypercorrection".
TLDR
The pronunciation of 'iron' in standard varieties of English is EYE-URN (BrE: /'aɪən/, AmE: /'aɪrn/) and not EYE-RUN (which is also a common pronunciation of 'iron' in some varieties of English) because of a very common process called Metathesis. It's defined as the transposition/rearrangement of letters, syllables or phonemes (sounds) in a word.
Explanation
There are some other words that show the same change; 'wasp' used to be waps, 'bird' used to be brid and 'horse' used to be hros, but they've changed over time. Why is that?
It's because of a very common process called Metathesis. /'aɪən/ is the metathesised version of (/'aɪrən/). 'Iron' commonly used to be pronounced the way it's spelt (/'aɪrən/), but due to metathesis, its pronunciation became EYE-URN (/'aɪən/). Other words such as horse, bird, third etc., reflect the change in spelling; however, 'iron' doesn't reflect that change probably because metathesis applied to it after the spelling was standardised.
Metathesis:
Metathesis is defined as ’the transposition/rearrangement of letters, syllables or phonemes (sounds) in a word’.
Examples:
- 'wasp' used to be waps (wæps) [transposition of p and s]
- 'bird' used to be brid [transposition of i and r]
{Historical Metathesis}
'mix' being pronounced /mɪsk/ rather than /mɪks/ [transposition of k and s]
'desk' being pronounced /dɛks/ rather than /dɛsk/ [transposition of k and s]
'modern' being pronounced /'mɒdɹən/ instead of /ˈmɒd(ə)n/ (US: /ˈmɑː.dɚn/)
'pattern' being pronounced /pætɹən/ instead of /ˈpæt.ən/ (US: /ˈpæɾ.ɚn/)
Another famous example from Shakespeare's 'The Tempest' is the figure of Caliban whose name originates from a phonological metathesis of /n/ and /l/ in 'cannibal'. [ThoughtCo]
/r/-metathesis
Words having /r/ + vowel sequences are more susceptible to metathesis than others. According to A grammar of Old English Phonology by Richard Hogg, ‘R-metathesis normally occurs when /r/ is followed by a short vowel and a dental or alveolar consonant, usually /n/ or /s/’.
'Iron' is an example of /r/-metathesis. It was probably pronounced /'aɪrən/ (EYE-RUN) at one point, but it got metathesised to /'aɪərn/ (EYE-URN). However, the spelling remained unaffected.
Other examples of metathesis of /r/ include:
- bird from brid (bridde)
- third from thridde (ðridde)
- horse from hros
The silent R in 'iron' in BrE
The reason why the r in 'iron' is absent in British English is because the r is followed by a consonant now (followed by /n/ in /'aɪərn/) and British English is non-rhotic, meaning the r is only pronounced when followed by a vowel. The same thing happened to 'bird', 'horse' and 'third' too (i.e. the r is followed by a consonant, so it's silent).
There are different types of metathesis, 'colonel' (pronounced KE(R)-NUHL /'kɜː(r)nl̩/) can also be said to be a product of metathesis. (See this answer on ELU for the spelling and pronunciation of 'colonel')
According to Wikipedia, the reason for ‘common speech errors’ is also metathesis.
Examples include:
- perscription for prescription
- interduce for introduce
- revelant for relevant
- foilage and foliage
References:
Best Answer
The preposition of has two forms. The weak form /əv/ and the strong form /ɒv/.
The spelling of this word is strange because it uses an F instead of a V. However, English spelling is always strange, so this is not very surprising!
We usually use the weak form of this word. We use the strong form when the word is stressed or when it appears without a Complement afterwards:
In the first example, the word of is followed by its Complement, the word you. We use the weak form here. In the second example, the word of is at the end of the sentence. There is no noun phrase or other Complement after the word. We use a strong form here.
In relaxed casual speech, the weak form of the word of may sometimes be reduced to a schwa, /ə/ or just to /v/: