differences between different noun forms of same adjective

Question

What is exactly the difference between nouning an adjective different ways? Some only have one form, but others have both. Examples:

• hilarious: hilarity vs. hilariousness
• virtuous: virtuosity vs. virtuousness
• clandestine: clandestinity vs. clandestineness

As an aside, what causes some to only have one form, like cheap --> cheapness, and what causes others to have two? And some have none, it seems. For example, accomplished --> * accomplishedness.

Answer 1

Suffixes -ness and -ity are roughly synonymous, but they have different etymologies.

There are two roughly synonymous suffixes, -ness and -ity, which are typically used for forming abstract nouns from adjectives, as in example below.

• generous + -ness = generousness

• generous + -ity = generosity

The first suffix, -ness, is etymologically native, while -ity entered the language as a result of contact with French during the Middle English period.

(..)

In addition, the meaning of words in -ity is often not entirely compositional, i.e., not deductible from the meanings of the base and the suffix. Thus, it is both phonologically and semantically more opaque than -ness (cf. Riddle 1985: 443–444; Aronoff and Anshen 1998: 246).

This was extracted from the article PRODUCTIVITY OF THE SUFFIXES -NESS AND -ITY IN 17TH-CENTURY ENGLISH LETTERS: A SOCIOLINGUISTIC APPROACH.

Answer 2

This article explains the -ness suffix rather well, and links to several other suffixes, several of which are often used to make an adjective into a noun.

As for accomplished, it is the past tense of the verb accomplish.