I was going to ask this in the Questions section, but everyone asks in esperanto and I'm not at that level yet.

Anyway, I'm trying to figure out something about transitive & intransitive verbs.

22. The verbs so far given have been intransitive verbs, expressing a state or an action limited to the subject, and not immediately affecting any other person or thing, as la knabo kuras, the boy runs. On the other hand a transitive verb expresses an act of the subject upon some person or thing; as, la knabo trovas — —, the boy finds — —.

The above makes sense to me. However, when trying to translate the following

they lie on the grass

I don't know whether it should be

Ili kuŝas sur la herbo

or

Ili kuŝas sur la herbon.

Specifically, I don't know if kuŝi is classified as a transitive verb or intransitive verb in this sentence because "on the grass" throws me off.

For example, I know that in "they run a marathon", "run" is a transitive verb with the direct object being "a marathon"; but if I said "they run on the road" would "the road" be the direct object? They don't run "the road", but rather on it.

Sorry for the long question, but this is really confusing to me.

"kuŝi" is intransitive. The action of lying down does not accept a direct object. "On the grass" is a place, and therefore an indirect complement.

An example of a transitive verb with "grass": "mi vidas la herbon".

As for your question, by adding an -n to "herbo" you specify movement. "Mi kuŝas sur la herbon" means, roughly, that you weren't on the grass, but now you lie there (you moved onto it). "Mi kuŝas sur la herbo" means that you are on the grass, lying.

In general the direct object is not introduced by any preposition.

Thanks for the quick reply!

This really helps me out a lot. Thanks!!