英文中双o何时发长音,何时发短音?foodfoottoothbookbookcasebookshelfwoodboot……这样双o的单词发音有什么规则?

it depends, no rules my dear...

字母“oo”读音歌
“oo”发[u:]最常见,非重音中要短念.
字母“k”前不能长,“好脚站木羊毛”短.
“血”与“水灾”真特殊,“oo”读[∧]细分辨.
“oo”加“r”读作[ ],“poor”读[ ]好可怜.
注：“好脚站木”即：good,foot,stood,wood
第二句也可以是：“d、k之前oo短,“foot”、“food”恰相反.
1.长音：bloom,boot,cool,foot,moon,root,school,soon,too,troop,room,zoo
2.弱读短：classroom,schoolroom,workroom,bedroom,boyhood
3.k前短：book,*,cook,look,shook,took
4.[ ]:door,floor