adj noun

There is no rules to change from adjective to noun. Learn some suffixes. Just memorize it.

-ness
happy – happiness, useful – usefulness, stubborn – stubbornness
dry – dryness, dark – darkness, great – greatness, gentle-gentleness
rude – rudeness, weak – weakness, cool – coolness, sad - sadness

-ity
able – ability, sensible – sensibility, reliable, reliability, severe – severity
mobile – mobility, possible – possibility, popular – popularity, pure - purity
capable – capability, liable – liability, toxic – toxicity, vital – vitality
probable – probability, productive – productivity, noble – nobility,valid – validity

-ty
safe – safety, sane – sanity, certain – certainty

-ce, -cy
prudent – prudence, brilliant – brilliance, just – justice, patient - patience
violent – violence, prominent - prominence
literate- literacy, fallacious – fallacy, accurate – accuracy, vibrant – vibrancy
competent – competency, democratic – democracy, private – privacy

-tion, -sion
attractive – attraction, protective – protection, perfect – perfection, moderate – moderation
precise - precision, passionate - passion

-th
wide – width, long –length, true – truth, filthy – filth, deep – depth, warm - warmth

-ism
professional – professionalism, optimistic – optimism, pacific – pacifism
pessimistic – pessimism, plural – pluralism

2011-05-14 03:50:14 補充：
Typo error:
There are no rules .... (not "There is no rules...")

沒有單一方法，但也有一些規則可參考：

stable, stability

generous, generosity
curious, curiosity

happy, happiness
kind, kindness
playful, playfulness
good, goodness

sorry, sorrow

genetic, genetics

但有些字，可同時作adjective 和 noun，例如：evil、sore。