唔明白be的擺位

ginger2011 has explained it well in the Opinion section.

'The powers that be' is a noun phrase referring to the people or group of people in actual power. The phrase is the subject of the sentence.

'Erased' in 'want people's memories erased' is a past participle. You can write either
'want people's memories erased'
or
'want people's memories to be erased'
but not
'want people's memories be erased'.

'Be erased' can be used in a sentence like the following:
The authority demands that the people's June 4 memories be erased.
It is the same as
The authority demands that the people's June 4 memories should be erased.

'Want people's memories are erased' is clearly wrong as you put two verbs in the clause.

The powers that be (phrase) - Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/The_powers_that_be_(phrase)
In idiomatic English, "the powers that be" is a phrase used to refer to those individuals or groups who collectively hold authority over a particular domain. It is ... LOL

2014-06-21 23:10:59 補充：
Long live Wikipedia and the garlic 粉腸!

2014-06-21 23:13:04 補充：
What the hell is 'ACTUAL POWER'?

2014-06-24 13:53:05 補充：
garlic2010 is forever right. He can form boring sentences with his links and stolen dictionaries fast. He is also a hypocrite who sends emails to his soulmates to vote for his answers.

HE SENT ME EMAILS TO ASK ME TO VOTE FOR "HIS" RUBBISH ANSWERS!

本人為應屆dse考生, 就讀傳統女名校,成績優異.
已於中文大學及香港大學醫科面試, 並已被澳洲及英國醫學院取錄 .
現等候放榜, 希望作私人上門補習.
有興趣請電郵至
[email protected]

聯絡後我會再提供成績證明文件以作確認.
價錢時間地點可再作商討

On the mainland, "the powers that be" want people's June 4 memories erased.
在內地，當權者企圖把六四從國人的記憶抹走。

In idiomatic English, "the powers that be" (當權者) is a noun phrase used to refer to those individuals or groups who collectively hold authority over a particular domain.