Postby Brody » August 8th, 2006 5:43 am
This is mostly conjecture, but I think it you use it when you need to emphasize the plural nature of the noun. Technically, you would never need to use it (you could use the singular in any grammatical situation and have it make sense, whereas in English you could not, ie "He go to the mall"); however it helps explain things and again, emphasize things.
I also think there are only certain nouns you use it with. Also, some nouns normally take ら instead of たち。I've had it explained to me that たち is more polite than ら, yet I'm not sure if that's totally true...there are some nouns that just sound better/more natural with ら over たち, despite politeness level.
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