niiden (englanniksi)

their (colloquial)
theirs (colloquial)
used instead of heidän (colloquial)
# their (colloquial)
used without possessive suffix in the main word owned (colloquial)
#: 'Niiden' äiti on 45-vuotias (colloquial)
#:: 'Their' mother is 45 years old (colloquial)
# theirs (colloquial / that which belongs to them, used without a following noun)
#: Tämä kirja on 'niiden'. (colloquial)
#:: This book is 'theirs' (colloquial)
# they (colloquial)
used with monopersonal verbs which require genitive such as pitää (colloquial)
täytyä (colloquial)
tulla (colloquial)
olla + passive present participle etc (colloquial)
#: 'Niiden' pitää (colloquial)
täytyy (colloquial)
tulee tehdä (colloquial)
on tehtävä tämä nyt. (colloquial)
#:: 'They' have to do this now (colloquial)
Sana perusmuodossa: ne / Sana perusmuodossa: niisi