英语在线等问题

正确。
如果后半句用将来时会更好：the fewer mistakes you will make.
相当于：If you are more careful, you will make fewer mistakes. 这种语法就是中学老师常常说的“主将从现”，即从句用一般现在时，主句往往用将来时。

我想是这样
The more you care about it, the more you are not likely to make mistakes.