rook gone

For beginners only:

Black to play. Indicate two moves (amongst other moves) that black MUST have played before.

Originally posted by Mephisto2
For beginners only:

Black to play. Indicate two moves (amongst other moves) that black MUST have played before.

[fen]rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBN1 b Qkq - 0 1[/fen]

Nxh1; Ng3?

Originally posted by fetofs
Nxh1; Ng3?

No. Why not?

The white knight on g1 could have come out and the rook could have been taken on this square?

Edit: meant to reply and quote, this was in answer to "why not?"

Originally posted by nickhawker
The white knight on g1 could have come out and the rook could have been taken on this square?

Edit: meant to reply and quote, this was in answer to "why not?"

Replace 'could have' with 'must have'.
Black is on the move, he made an even number of moves, all knight moves, allowing them to go back to their original position.
Hence, white made an odd number of moves, consisting of an even number of knight moves, and an odd (at least one) number of rook moves. The rook on a1 can only have made an even number of moves, hence, the rook on h1 must have made an odd number of moves, and was captured on g1.
The capture was Nh3xRg1 because the knight would have harassed the white king from f3. And the knight must have gone back via h3: Ng1-h3 for the same reason.

edit: typo's

Originally posted by Mephisto2
Replace 'could have' with 'must have'.
Black is on the move, he made an even number of moves, all knight moves, allowing them to go back to their original position.
Hence, white made an odd number of moves, consisting of an even number of knight moves, and an odd (at least one) number of rook moves. The rook on a1 can only have made an even number o ...[text shortened]... from f3. And the knight must have gone back via h3: Ng1-h3 for the same reason.

edit: typo's

I thought about that, but I didn't try enough...

Originally posted by fetofs
I thought about that, but I didn't try enough...

It's best on these type of problems to play out the game and verify the moves do happen.