When the object is ي (“me”), Arabic places a protective ن between the verb and ي:
ضَرَبَ + نِ + ي = ضَرَبَنِي
This nūn is called نُونُ الْوِقَايَةِ. Its kasrah suits the following yā. In ضَرَبَنَا (“he hit us”), نَا itself is the attached object; no extra protective nūn is added.
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Attached Object Pronouns in Verbal Sentences
When the object is ي (“me”), Arabic places a protective ن between the verb and ي:
ضَرَبَ + نِ + ي = ضَرَبَنِي
This nūn is called نُونُ الْوِقَايَةِ. Its kasrah suits the following yā. In ضَرَبَنَا (“he hit us”), نَا itself is the attached object; no extra protective nūn is added.