Mandaly, could you explain the rule, when to pronounce a hard "nn" as opposed to a nasal "en", such as in jén?

Sure.
If there is a double n or an accented letter before the n, then you will the "nn" prononciation, as in
kabann
lamàn
ijyèn
lapenn
reponn
sann
bann
pann
pàn
Otherwise it should bea nasal (non-resonating sound).
manman
tonton
lapen
san
ban
pan

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