when g become ng ?

[weal53005296]

when g become ng ?
like in
niGItaka
kyoo GI shitsu
is any grammer or role for g Pronounced ng

[Belton]

according to wikipedia

/ɡ/ may be weakened to nasal [ŋ] when it occurs within words — this includes not only between vowels but also between a vowel and a consonant. There is a fair amount of variation between speakers, however. Some, such as Vance (1987), have suggested that the variation follows social class; others, such as Akamatsu (1997), suggest that the variation follows age and geographic location.

so yes it happens but there aren't constant rules, it's a type of accent.
http://en.wikipedia.org/wiki/Japanese_p ... #Weakening