So you play the Ng5 line with white ? interesting choice, full of tactics.
Have you ever met the weird 4...Bc5 move ?
Well isn't that a blunder? your opponent doesn't even try to parry the threat to f7 !
Let me tell you this : if you're not prepared against this, and try to grab material, you will likely be crushed very quickly !
That's the problem with forcing openings like Ng5, you get wild positions full of traps and soon you get to reciting theory instead of playing chess! (i started that way too!)
So my advice : don't learn too much theory, try to grasp the main ideas, and if you play tactical and forcing openings, be aware of the most common traps !
Back to this strange 4...Bc5 move.
It's called the traxler counter attack, and it can lead to some of the most hair-raising lines from all chess openings !
The strategical idea (if we can say so from such a tactical opening) is this :
With his 4th move, white has played the same piece twice, breaking a general principle. So why not lure him into losing more time, even at the cost of a piece. or more.
if white forks and takes the rook, he has played Ng1-f3-g5-f7-h8, 4 moves to get a knight in a corner...needless to say, if black knows what he's doing, he can put these tempis to good use !
Here's an example of what can happen :
I like this game because black follows the logic of the opening to the maximum extent : he doesn't sacrifice just one piece for quick development and attack but his whole army, and the last remaining piece, the glorious black knight delivers mate !
How to avoid this kind of fate? not by learning loads of theory, just by avoiding the attack by not being too greedy !
A simple and effective way to meet this with white is (4.Ng5 Bc5) 5.Bxf7+ Ke7 6.Bb3 followed by d3, Be3, and 00, denying black of his attack by neutralising the strong bishop on c5.
Note : even after Bxf7+ it can be poisonous!
for instance in the line i just gave, black can play the tricky 6...d6 (6...Rf8 and 6...Qe8 are the other moves).
Black once again allows the fork on f7, but you should stick with my quiet d3 move.
A little exercise now : what would happen if you played the fork anyway ?
Try to find black's best move
Black to play