关于何时省略从句中的which、that、who等

关于何时省略从句中的which、that、who等
本来在百度里搜了这个问题,发现一位朋友的回答是：当被修饰的先行词做宾语时可以省略.并举例说the teachers who were inveted to the concert arrived on time中的who就不能省略.但是有两个问题：
1.All I need is diligence.中不就是all what I want is diligence的省略吗?
2.在chinadaily中看到一句话：She said the twice-yearly forum,launched by the presidents of the two countries,and the successful implementation of promises made at the first two sessions reflect efforts made by the two sides to settle economic disputes through negotiations.
其中 twice-yearly forum和the successful implementation of promises应该是reflect的主语啊,但是为什么文中写the successful implementation of promises made at the first two sessions而不是the successful implementation of promises which is made at the first two sessions?
小弟水平有限,这个问题确实不懂,希望懂的朋友能帮我解答,不要一大段语法书直接粘过来让我自己找,最好详细点,
lossprince你好，但是你可能没有看清我的问题，例子中并没说teachers不是主语，例子是说当被修饰的先行词做宾语时可以省略，如果先行词是主语的话不能省略，因为teachers是主语，因此who不能省略。我的问题是如果例子的说法是正确的，那么chinadaily上的那句话用法是和他矛盾的，我不知道正确说法应该是什么。你的建议很好，我的语法确实不扎实，我会买本书来看，但是如果你知道的话请帮我先解答一下我这个问题，我应该不至于看不懂，