Ng marks possession and always precedes a noun, like "Bibili ka ng kanin" [You will buy the rice]. Nang marks adverbs, which describe actions with adjectives and usually end with -ly in English, as in "Tumakbo ako nang mabilis" [I ran quickly].

*ng always comes before a noun.(verb-ng-noun) Ex. Kumain ako ng pananghalian. (I ate lunch) *Nang is use for a phrase. Ex. Nang umulan ng malakas. (when heavy rain falls.)

https://m.facebook.com/DepEd.Philippines/photos/a.380071542052546.85463.359105487482485/742240422502321/?type=1&source=57 read this ????, hope this helps