Their is play = They're playing a game. It is a soccer game.
Their is= They're good friends. I meet my friend, it was nice.

(NOTE: Se Yun, always remember that "they're" and "their" are diferent. Listed below are the exmample sentences that shows their difference:

They're very kind. ; They're very noisy. ; They're talking about our homework.

It is their house. ; They are cleaning their room. ; They are trying to fix their car. ^^)

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Hi Se Yun! ^^ Please remember the note that I've written on your homework. ;) You did a good job. :D And, I hope that you'll have a very fun long weekend. :D Enjoy the rest of the night, and see you on Monday! Good night! ^^

~Teacher Lady ^.^~