BBC news 2010-08-29 加文本

1.The report, covering the years 1993 to 2003, suggests Rwandan troops may have committed acts of genocide during fighting in the Democratic Republic of Congo.

may have committed对过去发生的事情表示猜测

1)may / might + have + 过去分词”表示对已发生的动作或存在的状态进行不肯定的推测，might的语气比may弱一点。这种结构主要用于肯定句和否定句，疑问句改用can或could。如：

They may not have known it beforehand.

You might have read about it in the papers.

2)“must + have + 过去分词”表示对过去发生的事情或状态进行推测，语气比较坚定，通常只用于肯定句。如：

It must have rained last night, for the ground is wet. 

You must have been mad to speak to the servant. 

3)“can / could + have + 过去分词”表示对过去某种情况的怀疑或不确定。can和could一般用于否定句和疑问句，could的语气较can弱。如：

I didn't see her at the meeting this morning; she can't / couldn't have spoken at the meeting.

He can't have finished the work so soon. 

4)“should / ought to + have + 过去分词”表示过去本该做某事但没做，其否定式表示过去不该做某事但做了，这种句式含有不满或责备之意，ought to的语气比should强一些。如：

You ought to / should have studied harder. 你本应该更努力学习的。（但没有）

He oughtn't to / shouldn't have done that. 他本来就不该做那件事。（可是做了）

5)“would ＋ have ＋ 过去分词”表示对过去的某种情况进行猜测，或本来要做某事却因某种原因未做成，通常用来说明某一情况，但不像用should或ought to那样含有责备之意。如：

I guess the poet would have been about twenty when she wrote her first poem.

Another worker wouldn't have acted like that.

2.Tens of thousands of people have taken part in a rally at the Lincoln Memorial in Washington led by a conservative talk show host, Glenn Beck, who once described President Obama as a racist who hated white people.

这句话的主干是：Tens of thousands of people have taken part in a rally。at the Lincoln Memorial in Washington是地点状语。led by a conservative talk show host是过去分词做后置定语，修饰rally。who once described President Obama as是定语从句，修饰Glenn Beck。

3.Opponents of the rally's organizer, charismatic conservative commentator Glenn Beck, have attacked him for holding the rally on the anniversary of Martin Luther King's famous 1963 "I Have a Dream" speech.

这句话的主干是：Opponents have attacked...of the rally's organizer是介词短语做后置定语，修饰Opponents。