什么时候用原形,什么时候用比较级,什么时候用最高级,急.ing!

比较级：
Taking bus is faster than riding bike.
Walking is slower than running.
You are taller than Jim.
She is shorter than Mary.
The park is more beautiful than before.
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最高级：
She is the most beautiful girl in the class.
This is the tallest buildling in GuangZhou.
Tom is the youngest student in our team.
That is the most delicious food in my life.
Yellow is the brightest colour in this picture.

没有比较对象，只是描述的时候用原形。－－我很年轻。I'm young.
想表达 是与某某（一个同类对象）相比，更为如何如何时，用比较级，常与THAN连用，有时THAN会省略。－－我比他年轻。I'm younger than him.
想表达在某某一类中（至少得是三个同样的对象才可算做一类）最为如何如何时，用最高级。－－我是班级中最小的一个。I'm the youngest in m...