Gokyo Shumyo, Section 1, Problem 21 / Solution

Main line  

White must force Black to connect with first, and then occupy the vital point with .

Mistake at  

If White plays the 2-1 first, Black makes an approach ko for the life of the group. Before winning the ko, Black has to connect the marked stones. Playing a provides three local ko threats for White.
(Note: While Black's ko threats have to be huge to win the ko, Black can show a profit if the threat that White ignores is large by comparison with giving up the two marked stones.)

Mistake at  

here fails to see the vital point. makes miai of a and b.

Mistake (ii) at 3  

Here too, White misses the vital point. makes miai of a and b.