which; what (way)
this
say; well; um; er
that; the
どの電車に乗ればいい。
Which train should I take?
どの道を通っても駅に着ける。
You can reach the station no matter which road you take.
どの選手が優勝するか最後まで分からなかった。
It was uncertain until the end which player would win.