回答如下：

【1】ahead of 在...前面，指具体和比喻空间上前面，还可以指时间上提前。

A still better tomorrow lies ahead of you.
展现在你们前面的是更加美好的未来。

The young man walked at the head of the parade.
那年轻人走在游行队伍最前面。

I think the work can be completed ahead of time.  )

我认为这项工作能提前完成。

Our tanks cut in ahead of the escaping aggressors.  

我们的坦克抢到前面去拦住那些溃逃的侵略者。

He soon got ahead of the other runners in the race.  

在赛跑时，他一到别人前面去了。

The car behind wants to get ahead of us.  

后面那辆车想超过我们。

He will be ahead of others in English.  

他在英语方面将超过别人。

The project was completed ahead of the schedule.  

这项工程提前完工。

Directly ahead of us is the royal palace.  

我们正前方就是皇宫。

A punctual person always finishes everything ahead of time.  

一个守时的人总是把事情提前做好。

A still better tomorrow lies ahead of you.  

展现在你们前面的是更加美好的未来。

They finished the work ahead of time.  

他们提前完成了工作。

I think the work can be completed ahead of time.  

 【2】in front of 一般用作表示具体的地点介词，可以指具体和比喻空间上前面，但是不可以指时间上提前.

I think I will put the desk in front of the window.

我想我会把书桌放在窗户前面.

There is a car parked right in front of our gate,and I can not get out.

一辆汽车就停在我们的大门前，我出不去了.

He was sprawling in the sofa in front of the TV.
他伸开手脚坐在电视机前的沙发上。

The car in front of me stopped suddenly and I had to brake.
我前面那辆小汽车突然停住，我只好刹车。

The speaker had a sheaf of notes on the desk in front of him.
演讲者在他面前的桌子上放着一叠笔记。

If you're phoning from outside London, dial 01 in front of the number.
从伦敦以外的地方打来电话，要先拨01再拨电话号码。

There is a tree in front of the house.
房子前面有一棵树。

He is mending the wire mesh in front of the chicken coop.
他正在鸡笼前面修理铁丝网。

The driver blew his horn when the child stepped in front of the car.
当小男孩走到汽车前面时，汽车司机按响了喇叭。

Stop showing off in front of everybody.
别再在大伙跟前卖弄了。 

【3】before 一般用在时间上，或者顺序

B comes before C in the alphabet.
B在字母表里排在C之前

Annie was born a few weeks before Christmas.安妮出生在圣诞节前几周。

Before World War II, women were not recruited as intelligence officers.

在第二次世界大战之前，女性不能当情报人员。

My husband rarely comes to bed before 2 or 3am.

我丈夫很少在凌晨两三点之前睡觉。

但是在以下几种情况下也可以用于地点之前

The man standing before the blackboard is our English teacher.   
站在黑板前面的那个人是我们的英语老师。

We got up before sunrise.   
我们在太阳升起之前起床。

Before crossing the road, you should have a look at both ways.   
过马路之前,你应该看看路的两边。

Quality comes before quantity.   
品质优于数量。

We must stop and rest before going any farther.   
我们必须停下来休息一下再向前去。

You must pay attention to this problem before every thing.   
你应该首先注意这个问题。

The criminal case comes before the court.   
那桩刑事案件已送法院审理。

Great sums of money were placed before the scientists by the government.  
政府为科学家们提供了大笔的经费。

What do you earn before tax?   
在未缴税前你的收入是多少?

He would die before betraying his country.   
他宁愿一死,也不肯叛国。

(1) 说明事物出现的顺序

B comes before C in the alphabet.
B在字母表里排在C之前

I think your name comes before mine.

我想你的名字排在我前面.

She knelt down before the fire to warm herself.

她跪在火前取暖.

He always put the interests of others before his own.
他总把他人的利益放在第一位

(2)   表示当着某人的面(in the presence of sb)

He made a statement before the House of Commons.
他在下议院当众发表了一个声明

I will not have you talking indecently before our guests.

我不允许你在客人面前讲不体面的话.

(3)   用在right before my eyes, before my very eyes短语中

He did such a thing before my very eyes.他在我眼前做了这样的事情。

情不自禁做某事  can't stop doing sth  还是  can't help doing sth.  或者两者均是？

 小数后面加可数名词单数还是复数？

1米  1 meter

2米  2 meters

0.5米  0.5 meter 还是 0.5 meters   ?

回答如下：

英语中单复数以数量1为标准，大于1用复数，小于等于1用单数

Everyone _____ has been to Shanghai says it is a modern city .

A.why        B.whose        C.who        D.which

标准答案选C.  但是先

回答如下：

答案为C，以上句子为关系代词who在从句作主语的定语从句

意思：去过上海的人多说它现代化城市

请问老师：ahead of与in front of 在表示空间或位置时，可以替换使用吗？

 ahead of与before在表示时间时，可以替换使用吗？

There are many difficult tasks in front of us . （OK）  

可以说成

We have many difficult tasks ahead of us. （OK）

这两句话可以作为同义句使用吗？

I think the work can be completed ahead of time. （YES）  

可以说成

I think the work can be completed before time.  （NO）   

这两句话可以作为同义句使用吗？

求老师解答，非常感谢！

继续回答如下：

(1)在表示具体的或者比喻上的空间，ahead of和in front of 两者都可以使用的，意思在....前面

Henry generally stayed ahead of the others in the academic subjects. 

亨利的文化课成绩通常都优于其他学生。

He was running 50 metres ahead of me .

At that moment two girls passed in front of me.

They say there is a great future ahead of me.

You have splendid life in front of you

=You have splendid life ahead of you.

I saw a man in a blue jacket thirty metres ahead of me.

我看见在我前面30米处有一个身穿蓝色夹克的男人。

She walked ahead of Helene up the steps into the hotel. 

她在海伦妮前面迈上台阶，进了旅馆。

I tried to think about all the problems that were ahead of me tomorrow.

我试图考虑明天我将面临的所有问题。

She spent all night thinking about the future that lay ahead of her.

她整夜未睡，满脑子都想着她未来的前途。

We have a very busy day ahead of us today. 

今天迎接我们的将是非常忙碌的一天

Millar finished 1 minute and 35 seconds ahead of Thierry Claveyrolat. 

米勒领先蒂埃里·克拉韦罗拉1分35秒完成比赛

The Prime Minister was speaking ahead of today's meeting. 

首相在今天的会议开始之前作了发言。

(2)但是在表示时间上提前，只可以使用ahead of不可以使用  in front of ，而且后面为不具体的时间名词,表示要比预先设置的时间提前发生或者完成的动作

This dish may be prepared a day ahead of time and refrigerated. 

这道菜可以提前一天准备好并冷藏起来。

The election was held six months ahead of schedule.

选举提前了6个月举行。

They reached their aims ahead of time (True 真确的)

They reached their arms in front of time(False错误的）

I think the work can be completed before time.  （NO 不真确的） 

The project was completed ahead of the schedule.  

这项工程提前完工

(3)before后面需要接表示具体的时间表示在....具体时间之前

They can finish the work before July.5.

他们可以在7月5日之间完成这项工作

My husband rarely comes to bed before 2 or 3am.

我丈夫很少在凌晨两三点之前睡觉。

（特别备注，ahead  adv. 可以单独使用用来修饰动态动词和静态动词，也可以作后置定语）

He has gone ahead to see if the road is clear.

他到前面去看道路是否畅通了.

He is ahead of the age.

他站在时代的前面.

Do you know the man ahead ?

你认识他们的那个人吗？

Go ahead!

请吧

Full speed ahead!

全速前进.

Brett looked straight ahead.

布雷特直视前方。

I peered ahead through the front screen.

我透过面前的挡风玻璃凝视前方。

The road ahead was now blocked solid.

前面的路现已被堵得严严实实。

We are moving ahead with plans to send financial aid.

我们提供财政援助的计划进展顺利。

Now BBC World Television is forging ahead on its own. 

现在英国广播公司的全球电视节目正靠自己的力量稳步推进

Australia were ahead throughout the game

澳大利亚队在整场比赛都处于领先地位。

Scotland were ahead in their European championship qualifier in Iceland... 苏格兰队在冰岛举行的欧洲杯预选赛中处于领先位置。

A goal would have put Dublin 6-1 ahead. 

只要再进一球，都柏林队将以6比1领先

The government wants figures that help it to plan ahead.

政府需要一些有助于其提前制订计划的数据。

Do book ahead as the restaurant is very popular. 

该饭店非常受欢迎，请务必提前预订。

Summer weddings need to be arranged months ahead. 

在夏季举行婚礼需要提前几个月作安排。

I went ahead and waited with Sean.

我先走的，和肖恩一起在那儿等。

You just go on ahead. I'll come by later.

你只管先往前走。我随后就来。

I'd have to send Tina on ahead with Rachael. 

我只有让蒂娜和雷切尔一起先行一步。