過(guò)去時(shí)態(tài)。ago用在過(guò)去時(shí)態(tài)中，before用在完成時(shí)態(tài)中。兩者都是以前的意思，對(duì)比起來(lái)，before主要用于過(guò)去完成時(shí)，ago主要用于一般過(guò)去時(shí)。ago不能單獨(dú)使用，應(yīng)與three days(months，weeks)等連用，而且和動(dòng)詞的過(guò)去時(shí)連用。

表達(dá)將來(lái)時(shí)的幾種結(jié)構(gòu):

01. be going to do

表示最近打算進(jìn)行的事,具有一定的目的性和計(jì)劃性.(這種安排既有可能是主語(yǔ)做出的,也有可能是別人安排 的)

We are going to have a meeting tomorrow.

表示即將要發(fā)生的事情，不可避免要發(fā)生的事情.(多用于表天氣)

It's getting cloudy. It is going to rain.

02. be to do

表示按計(jì)劃要發(fā)生的事,一般認(rèn)為,這種計(jì)劃是雙方約定的,而 be going to do 所指的安排不一定是雙方約定的.

We are going to meet them at the north of the bridge, because it is the only road they can follow. 我們打算去橋的北邊堵他們,因?yàn)槟鞘撬麄兊谋亟?jīng)之路.

We are to meet each other with them at the north end of the bridge. 雙方約定好要在橋的北邊見(jiàn)面.

be to do 還有很多情態(tài)用法,be going to do 沒(méi)有以下用法.

You are to wash your hands before every meal.(表需要=need)

As Warrent's parents,you are to set a good example to him.(表責(zé)任,義務(wù),相當(dāng)于,must should) Are we to clean the corridor,Mrs Dell? (表征求意見(jiàn)=shall)

The temperature is to drop soon.(表示可能性)

Would she get cross if I were to sail into her of lying?(表假設(shè))