Adjective formation
Adjectives are words that describe the qualities or states of being of nouns: enormous, doglike, silly, yellow, fun, fast. They can also describe the quantity of nouns: many, few, millions, eleven.
Some adjectives can be formed and identified by their endings/suffixes attached to the end of a word to form a new word or to change the part of speech of the original word. A suffix is added to the end of a word to change its meaning or to ensure the word fits grammatically.
For example, the verb read is made into the noun reader by adding the suffix -er. Similarly, read is made into the adjective readable by adding the suffix -able. 
Adjective forming suffixes:
-able/-ible                       understandable, readable, eatable / edible
-ful             beautiful, helpful, harmful
-less           homeless, jobless, useless
-y  cloudy, foggy, sunny, dirty
Sometimes there is a spelling change:
\(1\). words ending in consonant double the consonant before adding -y: sun- sunny, fog - foggy
\(2\). words ending in e, drop it, and add -y to the base word: shade - shady, scare - scary
\(3\). words ending in y, replace it with i: beauty - beautiful
Suffixes can also change meaning:
able/-ible + verb = can be done
Eatable is something that you can enjoy eating and can do it regularly.
You eat things that are eatable. Edible is something that is safe to eat.
ful = ful of
helpful = full of help
less = without  
useless = has no use