This document outlines key concepts related to languages in society, including diglossia, additive/subtractive contexts, language shift/maintenance, decline/resurrection, conflict, and nationalism. Diglossia refers to two languages used in one community for different purposes. Additive contexts see the addition of a second language enhancing the first, while subtractive contexts see the second replacing the first. Language shift occurs due to various social/political factors, while maintenance is encouraged by things like institutions in the native language. Conflict arises from struggles over political/economic power and issues of rights. Nationalism often favors one language over others as a symbol of identity.