hard working还是working hard,hard work呢

After a day of hard working/hard work,she felt tired.(Both 'hard working' and 'hard work' are fine but 'hard working' is better)
'hard working','hard work' 两者皆可用.但'hard working'较佳.
1....OF hard working,she felt tired.
'Hard working' is the object of the preposition 'OF'.
'Working' is a gerund.'Hard' is an adjective which modifies the gerund 'working'.
2....OF hard work,she felt tired.
'Hard work' is the object of the preposition 'OF'.
'Hard' is the adjective which modifies the following noun 'Work'.
3.After a day of working hard,she felt tired.(Wrong)
After she worked hard,she felt tired.(OK)
After -- conjunction.work -- verb.hard -- adverb.
After working hard,she felt tired.(OK)
After -- preposition.'after working hard' prepositional phrase modifies the subject 'she'.
Usually a person works hard.
'Day' cannot work hard.If you say 'a day of working hard',sounds strange.(i.e.'of working hard' used as a prepositional phrase modifies the noun 'day'.)
Hope this helps.
Cheers.

hard working
应该用表示动作的working

答案是 hard working
理由:参考后面的主要she

hard working
work 动词变成名词使用~~ 这里是辛苦的劳作
hard work 表示艰难的任务啊 ~~
It is a hard work. 这是个艰难的任务（工作）。

hard working（表示辛勤的工作）
因为hard word表示很难的工作

hard working.
这几个词的性质不同。
根据题意应该填入的是名词性质的词。只有hard working 符合。
你的理解没错，very good！

答案:hard working
hard working 是名词性的词组
而working hard,是动词词组
hard work 这个应该没有这个词组

hard working, hard work 均可。
只是working用分词形式作名词，更能突出工作这一行为，比用work生动贴切。

Although hard-working, he couldn't earn enough to support himself.
尽管努力工作，他的收入还是不够自己糊口。