Answering student's questions

 The position is from a game Pachman-Bronstein Prague 1946.

Q:

If  19.  f4   then 19.....h4!

The reply by White 20.gxh4 would be anti-positional for White.But how does Black then go about capturing the h pawn. Would  Black then play 21.....Bf6 ? In trying to capture the h4 pawn Black would take the pressure off d4.

A:

First of all, having sacrificed a pawn like this, the side not always hurries to take it back immediately on a following moves, it can be like a long-term sac, for example with intention to weaken that pawn and then it can be a long-term burden to opponent, which then can make his pieces to watch this pawn all the time and to be tied up with its defence, and in the meantime the side who made the sac can be active on any other part of board, having a superiority in forces and activity there. thus to defend this pawn all the time can be bad to opponent, and to give it back can also be not good. GMs often sac such pawns with composure and use positional advantage connected with it. But there is also one condition, when sacrificing it, that it is better that such pawn wasn't mobile then, because sometimes it can be possible to take and then to advance the pawn, like after Bf6 move that you considered W can do it. So it could be good to have N on f6 to prevent it from moving or smth like this, and in this case after the capture Bk has also to fight against this possibility as well. Well noted by you, that bf6 takes to many time to get this pawn back and can get a bit misplaced. Then Qd8 maneouvre followed by Qh4 can be good in this case, especially that White's K-side got weakened after this capture, along with f4 pawn move (like it was in some of your games, that the move f4 can be connecetd not always with initiative but also can weaken K-side sometimes). Thus Qd8 h5 Qf4 with double attack on f4 and h5.

Also it is very common to continue with attack in the centre after such a sac, and another alternative can be 1... Rxa1 2. Rxa1 Nce6 3. Nde2 Nxf4 and white is positionally broken, or 1... Rxa1 2. Rxa1 Nxb3, everything based on vulnerability of W's centre.