In what situations is it impossible to win for a player? For example due to not enough material or the composition of the material is not useful. I guess for example when you only have a king and a knight left or so and the opponent has far more pieces? I'm looking for the (few) hard cases where "you can't win". For example when you only have a king.
There are very few positions in which it is actually impossible to win although you are up material. Basically only KN vs K or KB vs K.
Then there are positions which can be won, but this would require such egregious errors by your opponent, that in practice a draw will inevitably be agreed. Examples are KNN vs K, KBB vs KB, KR vs KB.
And then there are theoretically drawn positions which give you very real winning chances even against strong players. KR vs KN, KRB vs KR, KQ vs KNN.
There are also known "fortresses" where the stronger side cannot break through once the weaker side has achieved a certain setup. Here is one famous (and relevant) example:
[White "The wrong"] [Black "bishop"] [FEN "k7/8/8/PK2B3/8/8/8/8 w kq - 0 13"]