Nouns are marked for whether they are considered
to be causing or undergoing an action.
They are said to be marked for either the
"ergative" or "effective" case.
The ergative case indicates that the
noun marked is causing the action,
and the effective case indicates that
it is the one undergoing the action.
For instance, "I" in "I ate the apple"
would be marked for the ergative case,
while "apple" would be in effective case.
The instrument used to perform an action
would also be marked in the ergative.
For instance, depending on how you look at it,
"stone" in "I threw the stone" could either be
ergative or effective: it would be ergative if
it was thrown to accomplish some action.
But in the case of something like "I was startled",
"I" would be marked in the effective case,
as it is not causing but undergoing an action.
This differs from languages like Russian and Japanese,
where in both "I ate the apple" and "I was startled",
"I" would be marked in the "nominative case".
"Apple would be marked in the "accusative case",
and in "I write with a pencil", "pencil" would be
marked in the "instrumental case".