Take previous element from array if condition on current element matches
This code would benefit from a previous variable...... and it would also benefit from being real code, not this hypothetical example..... This code is also really short, so it's hard to simplify more.

Still, using a foreach is better than the indexed iterator, and the logic is more obvious with named variables, rather than indexes... so:

private object[] array;
public abstract bool Condition(object o);
//...
private object FindStuff()
{
    object previous = null;
    foreach (object current in array)
    {
        if (Condition(current))
        {
            return previous;
        }
        previous = current;
    }
    throw new ItemNotFoundException();
}