He wants to be a great () star like Liu Xiang.用run的正确形式填空.为什么不是runer,而是running?拜托讲仔细,

Here your focus is on the word "star". So "running" describes what kind of "star" you are talking about. An adjective, therefore, is to be used.
However, we don't usually call runners "stars", so it'll be more appropriate to use "runner" instead of "running star". "Running star" sounds very awkward. Actually, a better word would be "athlete".

running 现在分词做形容词

我觉得应该是 to be a great star of runer
但是一定要在那填就应该填一个 star的定语 因为star是名词 所以应该是现在分词作定语running

running
名词奔跑或形容词
runer
跑步的人，加er一般就是……的人。

runer,He wants to be a great （runer） like liu xiang,runer 已经是名词了
running现在分词做形容词,

running是现在分词作定语。

弄清他想成为star 不是runner
所以应该是修饰star 用形容词running