following are general rules in Tamil grammer....

Tamil grammer is based on the time it takes to say it.

kuril is the short form as "a" in "as"
nedil is the long form as "a" in "ass"


a kuril sound takes one second and a nedil sound takes two seconds.

a consonent followed by a vowel should either be a kuril or a nedil.
vowel should either be a kuril or a nedil
consonet, vowel and consonent should either be a kuril or a nedil.