"far"的比较级和最高级分别是 farther/further和 farthest/furthest。当"far"用来描述具体距离时，比较级是"farther"，最高级是"farthest"。当"far"用来描述抽象概念时，比较级和最高级则分别是"further"和"furthest"。例如：
1. She walked fartherthan her brother.
2. They traveled furthestto reach the summit.
3. In her thoughts, she went furtherthan ever before.
建议根据具体的语境选择合适的词形。