1. Indefinite a/an = one of a class; first mention: "A complaint was filed."
2. Definite the = already identified; unique; shared knowledge: "The complaint alleges fraud." / "the Constitution of Nepal" (there is only one); "assign the contract" (the contract already mentioned).
3. Zero article (–) = general/abstract plurals and uncountables: "– Courts interpret – statutes"; "– evidence was insufficient" (evidence is uncountable — never "an evidence" or "evidences").