Through out the whole song it goes, Em, C, D, G. These 4 chords get repeated 
through out the song. Standard tuning: E,B,D, G, A, E. 
For example:

Em                   C
There's a place we know,
D                          G
What's cold enough won't grow
Em                C
We have seen the dark
 D                          G
and the darkness took its toll 



This pattern continues throughout the whole song. 

The strumming pattern is: D-U-U-D-U-U-D-U-U-D
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