Past simple or present perfect?

The tense is present perfect progressive.

进行时)(现在,过去,将来,现在完成-进行时 )

Sum up the new vocabulary in this Unit, and Present Perfect Tense.

明确本单元所学的词汇和语法,增强他们正确操作的能力.

Explaining some topic already obtained uses present perfect tense.

说明某个课题现在已经取得的成果,适宜采用现在完成时.

Explain and illustrate the main use of the present perfect tense.

解释并举例说明现在完成时态的用法.

Rewrite the following sentences using the Present Perfect Passive Voice.

用现在完成时的被动语态改写下列句子.

The present perfect tense emphasizes the effect of the past action on the present.

现在完成式强调过去所发生的事情对现在的影响.

The molecular separator is present to remove most of the carrier gas.

分子分离器可除去绝大部分载气.

When it came to Anna he was stumped. What would he present to her?

轮到安娜时,他犯愁了.他将赠送她什么呢?

I assumed that the whole family would be present to watch me perform.

我暗地里想,全家一定都会去看我演出.

All those present to listen to her sad story were moved to tears.

凡在场听到她悲酸身世的人无不感动得流泪。

A vivid picture is present to our eye.

一幅栩栩如生的画面出现在我们眼前.

A vivid picture was present to his mind.

一幅生动的画面出现在他的脑海里.

Enough electrons are present to maintain electrical neutrality.

存在着足够多的电子以维持电中性.

She sniffed at the present to show her contempt.

她对这礼物嗤之以鼻以表示她的轻蔑.

Create other conversations for the people in the picture. Present to the class.

为图片里的人们写一些新的谈话内容. 在课堂上表演.

Each child brought a little present to the mother.

每个孩子给母亲带来一件小礼物.

Up to the present day no bond exists between the earth and her satellite.

直到现在为止,地球和它的卫星之间还没有建立任何直接联系.

Even by present-day standards these were large aircraft.

即使按照现在的标准，这些飞机也挺大的。

All optical network is the tiptop form of optical fiber communication at the present day.

全光网络是当今光通信的最高形式,它的普及关键在器件.

We have been making cars from the 1940's up to the present day.

我们自一九四零年到现在都在制造汽车.