Avoid Queen and Bishop Mate

How do I avoid this checkmate?

http://www.chess.com/livechess/game?id=1083072455

When black played Bb7 you should have recognised the battery lined up on the g2 square and that your knight was in great danger.

18 f3 would have prevented mate/loss of knight.

Bishop and queen are pointing there

And yah opened it up...

At that point the only thing you could do its sacrifice che knight for 2 pawns:

but your mistake was done before this move. When black did Bb7 you should immediatly spot the bishop adn queen allined to g2. Before doing your move ALWAYS check what your opponent moved and try to understand why he did that move.

Thanks!  This helped a lot

f3 is always an option, but the f-pawn shouldn't be moved most of the time, so just protect the g2 pawn

In his position 18. f3 was the only option. How do you propose he protect the g2 pawn otherwise ?

I was saying before he got into g2 trouble, or just do what I do, fianchetto the bishop