三个原子在一起,先不要管它们是不是排成一条线. 把HCN看成三角形.
C
/ \
/ \
H O
bending one will be:
C
/ \
/ \
H-> <-O
in which H and O atoms will move toward with each other.

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Bending two will be
^
|
C
/| \
/ V \
H O

in which, the equilibrium position of C atom, H and O will keep in the same plane. only C atom vibrates up and down.
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Finally, I think the linear structure is very misleading