Find the next letter in the series: B, Y, D, W, F, U, H, ?

The letters in the odd places are B, D, F, H — moving forward 2 steps each time. The letters in the even places are Y, W, U — moving backward 2 steps each time (25, 23, 21). The missing letter sits in the 8th place, an even place, so it follows U: 21 − 2 = 19, which is S.