"They could have won the race" means it happened in the past :). "They could win the race" means it is going to happen in the future , so sentence 2 is incorrect, it would be: "They could win the race, if they try hard enough"

Hello, Your first sentence talks about a race that happened in the past. they didn't win because they didn't try hard enough in the past. The second sentence would be talking about the possibility of winning a future race, but you would need to change the second clause. 'They could win the race if they try hard enough'. or 'They won't win the race if they don't try hard enough.' I hope that helps. Christine