Why is the N silent in "solemn" but pronounced in "solemnity"

Question

• Solemn → /ˈsɒləm/: It has only /-m/
• Solemnity → /səˈlɛmnɪti/: it has both /m/ and /n/ (/-mn-/)

Looking up their etymology didn't help much. But here is what etymology dictionary says:

• solemn:

  ... from Old French solempne (12c., Modern French solennel) and directly from Latin sollemnis "annual, established, religiously fixed, formal, ceremonial, traditional," perhaps related to sollus "whole" (from PIE root *sol- "whole, well-kept").
  [Etymonline]

• solemnity

  c. 1300, "observance of ceremony," from Old French solemnite, solempnete "celebration, high festival, church ceremony" and directly from Latin solemnitatem (nominative solemnitas) "a solemnity," from sollemnis (see solemn). Meaning "state of being solemn" is from 1712. Related: Solemnities.
  [Etymonline]

I guess it's because "solemn" is directly from Latin "sollemnis"? And "solemnity" from Old French "solemnite"? But I am unsure. Can anyone explain why the N is silent in "solemn" but not in "solemnity"?

Answer 1

The -n of solemn is silent because English no longer allows certain consonant clusters at ends of syllables. Wikipedia gives some info here. Basically, compared to what the pronunciation might once have been, or what the spelling might suggest, the consonant cluster is reduced -- one of the consonant letters is silent.

This phenomenon of reduction occurs only because of how hard it would be to pronounce such a cluster at the end of a syllable. But pronouncing the same sequence of consonants might be possible (and allowed in English) if at least one of the consonants is at the start of the next syllable. That is the case with "solemnity", where we can pronounce the /n/ at the start of the third syllable. This explains the pronounced /n/s in words such as condemnation, hymnal, columnist and autumnal (suggested by Steve).

There is more on this in this answer.

Mind you, the inflection suffix -ing doesn't block the reduction, so the "n" in condemning is still silent.

Answer 2

Broadly speaking, and not without exception, consonants need a vowel to make them pronounceable. It is very hard for English speakers to pronounce two consonants, especially nasals like m and n, together. (Some consonant pairs we do fine with such as Christmas, rhythm or lips.) But if you add a vowel afterward you can split it into two syllables making it easier to pronounce. So sol-emn, but sol-em-ni-ty. (Compare also with Solomon, sol-o-mon, which is easy to pronounce because of the intervening 'o'.)

FWIW, in my opinion, in some English accents you can hear the tiny remnant (rem-nant) of an n at the end of solemn.