10. If he had known that the river was dangerous he (not try) to swim across it.

10. Это условное предложение третьего типа (Third Conditional). Если бы он знал, что река опасна, он бы не попытался переплыть ее.

Правильная форма глагола: would not have tried.

Предложение целиком: If he had known that the river was dangerous he would not have tried to swim across it.

Ответ: would not have tried