Technically, nu is a non-focus marking particle. Meaning, nu is a particle that precedes a nouns when it's not the "focus" of a sentence/clause, whereas i/si mark the "focus" of the sentence/clause. Nu often occurs as ni, a contraction of nu + i; nu also occurs as the particle as when the non-focus is a proper noun (i.e.: when the particle si would otherwise be used). I'll try to explain it in a little more detail.
In any Chamorro sentence there must be a focus, which we mark with the articles i/si. For example:
- Hu ina i chalan. = I lit the road.
- Ha sångåni i ga'chong-ña. = He told his friend.
- Ma nå'i i patgon. = They gave the child (something).
- Manmåsångåni i famagu'on. = The children were told.
- Mågåggao i nana. = The mother was asked.
- Måleffa si Maria. = Maria forgot.
- Hu ina i chalan ni kandet. = I lit the road with the light.
- Ha sångåni i ga'chong-ña ni bidå-ña. = He told his friend about his deed.
- Ma nå'i i patgon nu regålu. = They gave the child a gift.
- Manmåsångåni i famagu'on ni estoria. = The children were told a story.
- Mågåggao i nana nu kande. = The mother was asked for candy.
- Måleffa si Maria as Jose = Maria forgot about Joe.
In example 2 "his friend" is spoken to, and the "deed" is being talked about. So, the "deed" is the non-focus of the verb sångåni.
In example 5 the "mother" was asked, so she is the focus of the verb mågåggao. The non-focus of the verb is "candy," since it explains what she was being asked for.
Lastly, in example 6 "Maria" forgets, and we see that "Joe"explains who she forgot.