我翻了一下室友的文法书(Understanding and Using English Grammer---Longman published)在其中一章有提到:
(书上的解释) (a) Adverb Clause: While I was walking to class, I ran into an old friend. (b) Modifying Phrase: While walking to class, I ran into an old friend. ---------------------------------------------------------------------------------------------------------- Omit the subject of dependent clause and the be form of the verb, as in (b)
(c) Adverb Clause: Before I left for work, I ate breakfast. (d) Modifying Phrase: Before leaving for work, I ate breakfast. ---------------------------------------------------------------------------------------------------------- If there is no be form of a verb, omit the subject and change the verb to -ing, as in (d)
Conclusion: An adverb clause can be change to a modifying phrase only when the subject of the adverb clause and the subject of the main clause are the same. A modifying adverbial phrase that is the reduction of an adverb clause modifies the subject of the main clause.
(我小小的看法) 换言之, 在(a)和(b)"be form of verb" 应指 was walking (请把两个看成be form of verb) 意思上主句的动作和从句的动作发生有同时性的关系 (was walking) 回过头看原问题, 如果还原一下看问题: When she was in Rome, she fell in love. 这句的主句动词为fell in love, 但是从句的意思却是表达一种状态 ( she was in Rome), 所以我认为不可省略. 或者直接变成 She fell in love in Rome会比较concise.