A/an - Singular, countable, undefined items.
"I have read a book." What book? Not clear = a.
The - Defined nouns + things that there is only one of.
" The spring of 2008 was warmer than any other. "
What spring? The spring of 2008 = defined.
No article - Nouns referring to general concepts (e.g. life, water, art etc.) that are not defined, geographical features (streets, mountains, towns, cities, lakes), the purpose of buildings.
" Life flourishes in spring". "I'm going to university"
Life and spring are concepts, not defined further = no article.