the ng in bantu languages is a nasal where to my ears it sounds like nyeh-nee, but the e is probably a long vowel, which would make it nyay-nee. with Ndereba, the n should be heard as a kind of grace note. en-dereba, but the en sound is very short. You'd probably have more luck being understood if you just said dereba. I worked on the n as grace note for two years and was still misunderstood while another guy just said the words without the n and was immediately understood.