(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[9]KM[0.00]
AW[bg][cg][dg][eh][fh]AB[bf][cf][df][eg][fg][gg][gh]LB[ag:A][fi:B]C[This corner is ko, but your move will not bring the corner to ko. It will simply result in white living.

There is a general rule: First limit the eyespace, then play the vital point. The moves to consider first are A and B.]
(;B[ci]C[This is not the move.]
;W[di]C[Because this move does work to secure life for white. Black C2 fails from here.]
(;B[ch]C[This looks good. But...]
;W[bi]C[Now what does black do?]
(;B[bh]
;W[ah]
;B[ai]
;W[ag]C[This is not a ko. White just backs off and is alive.]
(;B[fi]
;W[dh]
;B[af]
;W[bi])
(;B[bi]
;W[fi]))
(;B[ag]
;W[bh]C[This is also alive.]
(;B[ah]
;W[fi])
(;B[fi]
;W[ah]))
(;B[ah]
;W[bh]C[Again, alive.]))
(;B[bi]C[The 2-1 points are surprisingly ineffective.]
;W[ag]
;B[ah]
;W[fi]
;B[ch]
;W[bh]
;B[af]C[This is not a ko.]
;W[dh]
;B[gi]
;W[ai])
(;B[ah]C[This 2-1 point isn't much better.]
;W[bi]
;B[bh]
;W[ch]
;B[ag]
;W[fi])
(;B[ag]
;W[bi]
;B[bh]
;W[ch]
;B[ah]
;W[fi]))
(;B[ag]C[How about this?]
(;W[ah]C[If White plays normally.]
;B[bi]
;W[bh]
;B[di]
;W[dh]
;B[fi]
(;W[ci]C[White needs to play here. Now it is ko.]
;B[ch])
(;W[ei]C[This fails.]
;B[ci]C[Because this is only one eye.]))
(;W[ci]C[If white backs off to here.]
;B[bh]
;W[ah]
;B[ai]
;W[ch]
;B[ah]
;W[di]
;B[fi]C[White dies.])
(;W[di]C[This will lead to a ko.]
;B[bh]
;W[ch]
;B[fi]
;W[ah]
;B[ai]
;W[bi]C[Now it is ko - unfavorable for white.])
(;W[bh]
;B[di]
;W[ah]
;B[bi]C[Reverts to the same as before.]))
(;B[fi]C[This move will also lead to a favorable ko if black plays it right.]
;W[di]
(;B[bi]C[This does not work.]
;W[bh]
;B[ah]
;W[ag]C[This just lives for white.]
;B[af]
;W[ci])
(;B[ag]
;W[ah]
;B[bh]
;W[ch]
;B[ai]
;W[bi]C[This is the same ko as before.])))