Why some points cannot be played

First remove opposing stones with no liberties, then check your own. If capturing gives your stone a liberty, the move is not suicide. Being surrounded at the instant of placement does not by itself make a move illegal.

Sometimes each side can capture a single stone back, creating ko. This game prohibits immediate recapture that restores the position before the opponent’s last move. Play elsewhere first, wait for the reply, then check whether recapture is possible. The opponent may also connect the ko meanwhile.

A move elsewhere is not useful just because it is elsewhere. Why an opponent might answer, and when a ko is worth fighting, are covered in “Ko threats and ko for life.”