您好
过去分词置于句首做状语，通常表示被动或完成，说明谓语发生的背景或情况。
例1. ___ automatically, the e-mail will be received by all the club members.
A. Mailed out B. Mailing out C. To be mailed out D. Having mailed out
答案选A. 分词短语作状语，相当于一个假设
条件状语从句If it is mailed automatically, e-mail与mail out 是被动关系。
例2. __ for the breakdown of the school computer network, Alice was in low spirits.
A. Blaming B. Blamed C. To blame D. To be blamed
答案选B.
非谓语动词形式作原因状语，主语是blame所表达动作的承受者，因而用过去分词blamed.
过去分词置于句首还有一种比较特殊的结构, 即“be+过去分词+介词”结构表示状态的短语变化而来，也就是“be+过去分词+介词”短语置于句首表示状态时, 可以直接省略掉
be动词.
例3. __ with so much trouble, we failed to complete the task on time.
A. Faced B. Face C. Facing D. To face
答案选A. 可以将该分词结构看作是be faced with 短语置于句首作原因状语的省略。
同现在分词作状语时前面可以加when, while, though, if 等连词的情况一样，过去分词前也常加一些连词。
例4. When first __ to the market, these products enjoyed great success. (04 四川高考-22)
A. introducing B. introduced C. introduce D. being introduced
答案选B.非谓语动词短语相当于when these products were first introduced to the market作状语的省略。