test cases, "when", "what", "why"?

being new exam formed development, doubt bugging me. many too much? should tested, should tested, since should tested? examples given c# nunit, nonetheless i assume doubt itself libel agnostic.

here twin tide examples own, tests general list vigilant (being tested strings, initialisation duty adds 3 equipment {"foo", "bar", "baz"}):

[test]
public vacant countchanging()
{
    assert.that(_list.count, is.equalto(3));
    _list.add("qux");
    assert.that(_list.count, is.equalto(4));
    _list[7] = "quuuux";
    assert.that(_list.count, is.equalto(8));
    _list.remove("quuuux");
    assert.that(_list.count, is.equalto(7));
}

[test]
public vacant containsitem()
{
    assert.that(_list.contains("qux"), is.equalto(false));
    _list.add("qux");
    assert.that(_list.contains("qux"), is.equalto(true));
    _list.remove("qux");
    assert.that(_list.contains("qux"), is.equalto(false));
}

the formula sincerely self-commenting, i won't what's happening, nonetheless arrange thing holding too far? add() remove() tested seperately course, turn should i sorts tests? should i even have sorts tests?