After black plays Nc6-b4 (threatening white's bishop on d3), I don't understand why Qd1-a4 is not a reasonable response. (It checks the king and threatens the knight--the knight must retreat to block check, or be lost)
Attempting it gets a generic "Save the bishiop or weaken e4."
AKO007 is the lesson #. Lesson title is the topic title.
1. e4 e5
2. nf3 nf6
3. Nxe5 d6
4. Nf3 Nxe4
5. d4 d5
6. Bd3 Nc6
7. O-O Be7
8. c4 Nb4
Attempting 9. Qa4 gives "Save the bishop or weaken e4" with no further explanation. Forcing the knight to retreat with a pin on it would seem to accomplish this. (Though in thinking it through some more, it may not work as well as the correct move, an explanation would be good, I think)
Not actually threatening the knight, the bishop is defending it. Black can just play Bd7 and force the queen away.
Ugh, tunnel vision. Totally forgot about the bishop guarding the knight.