When is "tanween" (nunation) applied to a noun and when is it not?

Tanween (nunation) is affixed to declinable nouns that are neither in a genitive construction (mudaf) nor defined by "al" (the definite article). It is removed when the noun is in a genitive construction or defined by "al". Examples include the word "rasul" (messenger): "rasulan minkum" (a messenger from among you) (with tanween), "rasul Allah" (the Messenger of Allah) (tanween removed due to genitive construction), and "ar-Rasul ballagh" (The Messenger conveyed) (tanween removed due to definition by "al"). Tanween may also be affixed to some indeclinable nouns to indicate their indefiniteness, such as "sah" to denote silence from all speech.