It is an invariable particle which precedes the verb it modifies. So for example the following sentence:
pileh qappakas
I eat fish.
Turns to:
pileh wan qappakas
I need to eat fish.
Just as easy. The question now is how to create a sentence of the likes of "I need you to eat fish", in this case we would change the flow of the verb (the suffix) but include a possessive pronoun preceding the particle. So:
pileh ikanwo wan qappadas
I need you to eat fish.
The meaning here that it is one's need that the other person do the action, note that the flow is quite simple "you eat fish", and the possessive pronoun would be modifying the particle, the needing of something. So it is my needing that you eat it.
shinwo wan shain tanakash
I think he needs me to speak to them.
This one is more complicate, but the analyses is the same. It is his need / I speak (to) them. Notice how the object pronoun in this case can't be confused with the possessive as the possessive will be preceding the particle, leaving "shain" only to mean "to them". The transition of the verb marks a flow from a 1st person singular to some other person, which is being explicitly marked by the pronoun in this case.
A simple lesson for today and one more particle to get to know.