Today’s grammar lesson is on a special use of Japanese particle を (o).
You may already know that the primary function of particle を is to mark the direct object of a transitive verb. The direct object is the one who receives the action which is done by the subject. In other words, it answers to the question what or who about an action verb.
For example: わたしは、おもしろい本を よみました。
If you just say わたしは、よみました, you would ask なにを よみましたか？ The answer would be おもしろい本 を よみました。
Therefore おもしろい本 is your direct object.
However, some motion verbs that are intransitive can also take particle wo to mark the route of one’s movement.
Some of these verbs include あるく、およぐ、はしる、のぼる etc.
For example:
わたしは、公園（こうえん）を 走（はし）ります。
I run in the park
森（もり）をあるくのが、好（す）きです。
I like walking in the forest
さかなが、海（うみ）を およいでいます。
Fish is swimming in the sea
You may wonder why particle で which usually marks the place of an action is not used here.
Actually you can also use で but it may give a slightly different nuance.
If you say 公園（こうえん）で 走（はし）ります, then it literally means “I run in the park”. It means that your action of running takes place in the park and completes there.
Whereas 公園（こうえん）を 走（はし）ります literally means “I run the park” but it means more like “I run through the park to move from one point to another”. So を here is marking the route that is taken while following this movement.
So it would sound rather strange if you say ランニングマシーンを はしりました。
ランニングマシーンで はしりました。
Because when you run on a treadmill, you are doing the action of running in one place, not actually moving from one point to another.
That’s why the motion verbs such as とおる、わたる can only take particle を because by passing through or crossing, you are forced to move from point A to B.
Let’s compare these 2 sentences:
こどもが、プールで およいでいます。
さかなが、うみを およいでいます。
In the first sentence, A child is swimming in the swimming pool. プール is a place with limited size and people usually swim in it or swim back and forth inside the swimming pool not necessarily aiming at reaching a certain goal. Here, reaching from point A to B is not the point of focus.
In the second sentence, Fish is swimming in the sea うみ is a vast nearly-unlimited place compared to a swimming pool and fish are not confined to one area of the ocean but they freely move to different places without staying in one area. So in this context, を particle can be used.
So that’s the basic difference between を and で
で marks the place of an action
を marks the route of one’s movement
I hope this lesson was helpful. Please let me know by comments if you have any question.