across 指“从……的一边到另一边”，强调横过，多与动态动词连用。
along 指沿着一条直线在水平方向上运动。一般表动态，有时也与静态动词连用。
over 常和动态动词连用，指“从……上面越过”或“横越”；表静态时，指“在彼处”。
through 侧重从一端穿到另一端。
参考例句
1. He thought more and more along that line. 他根据那个原则想了又想。
2. Each careful step along the by way. 孤身上路时小心的走每一步。
3. He simply played along. 他只是独自玩玩。
英英解释
adv.
1. with a forward motion
2. in accompaniment or as a companion
3. to a more advanced state
4. in addition (usually followed by `with')
5. in line with a length or direction (often followed by `by' or `beside')
