They're so many uses of NG in tagalog. NG can be OF/BY for example. ang libro ay ginagamit NG aking kapatid/ The book is being used BY my brother. Ang libro NG aking kapatid ay sobrang mahal. The book OF my brother is very expensive. Other say, can be used as WITH.. But I cant give you an example for that since I dont know how to use NG as WITH. =P

In addition to JcieL's explanation about the usage of "ng" as a preposition / prepositional phrase, "ng" can also be used as an article such as: "kumuha ka NG isa'ng baso na tubig." ('get A glass of water.') note: the word "isa'ng (one)" here is a quantifier of the noun "baso (glass)". the article "ng" in Tagalog always goes before the quantifier (but not in English). "ama NG anak." ('father of THE son.') as for "ng" to be equaled to the preposition "with" in english language, you can say: "ipagpalit NG manika." ('exchange WITH doll") :)