ago/before/formerly/previously.导航词义：以前

1. ago:adv. 以前.指从现在起往前推算一段时间，通常和一般过去时的动词连用。

He left a moment ago.

他刚走。

There was a pond here three years ago.

3年前，这里有一个池塘。

2. before:adv. 先前，以前.指过去某时间之前的时间或泛指以前，通常与动词的现在完成时连用，有时也可与动词的过去完成时或一般过去时连用，也可用于具体的一段时间；在间接引语中用 before 而不用 ago。

Have you heard about it before?

你过去听说过这件事吗？

I have never met him before.

我以前从未见过他。

He said that he had met the man somewhere before.

他说他以前在什么地方见过这个人。

He was in Italy the week before.

他前一个星期在意大利。

3. formerly:adv. 以前，从前.泛指以前，有时常用于对比。

Formerly, Ted often lingered around a park, with nothing to do.

从前，特德时常在公园里闲逛，无所事事。

He was formerly a worker, but now he is a manager of a shop.

他曾经是个工人，可现在他是一家商店的经理。

4. previously:adv. 以前，先前.与 before 用法相同，但更加正式。

I haven't met him previously.

我以前没有见过他。

She previously worked as a teacher.

她过去当过教师。

Six months previously he had quitted his job.

6个月前他辞职了。

全部

1·His statement ran athwart what was previously said.

他讲的话与先前所说的相抵触。《牛津词典》

2·The contract was awarded to a previously unknown company.

该合同给了一家先前不知名的公司。《柯林斯英汉双解大词典》

3·Guyana's railways were previously owned by private companies.

圭亚那的铁路先前为私营公司所拥有。《柯林斯英汉双解大词典》

4·Radiation may damage cells in a way that was previously unforeseen.

辐射也许会以以前未预见的方式损坏细胞。《柯林斯英汉双解大词典》

5·I should like to add a corrective to what I have written previously.

我想对我先前写的内容进行补充修改。《牛津词典》

6·Marine life was discovered at depths previously thought to be azoic.

在先前被认为没有任何生命迹象的海洋深处发现了海洋生物。《牛津词典》

7·He may suddenly take a dislike to foods that he's previously enjoyed.

他也许会突然讨厌他以前喜欢吃的食物。《柯林斯英汉双解大词典》

8·A previously unknown extremist group has said it carried out Friday's bomb attack.

一个先前未知的极端分子组织声称其实施了周五的炸弹袭击。《柯林斯英汉双解大词典》

9·Women's sports are breaking down the barriers in previously male-dominated domains.

女子体育运动正在打破障碍进入一些先前由男子占主导地位的领域。《柯林斯英汉双解大词典》

10·The drug is to be reclassified after trials showed it to be more harmful than previously thought.

试验显示这种药比先前想象的危害更大，要重新归类。《牛津词典》

以前

1·He may suddenly take a dislike to foods that he's previously enjoyed.

他也许会突然讨厌他以前喜欢吃的食物。《柯林斯英汉双解大词典》

2·This means that we have to set all the previously mentioned properties.

这意味着我们必须设置所有以前提到过的属性。

3·One of the easiest ways to segment a market is to magnify gender differences or invent them where they did not previously exist.

细分市场最简单的方法之一是放大性别差异，或者在以前不存在性别差异的地方创造性别差异。

4·Steamboats were not only faster but also transported upriver freight for about one-tenth of what it had previously cost on hand-propelled keelboats.

蒸汽船不仅速度更快，往上游运送货物的成本也只有以前手推运货船的十分之一。

5·Japanese males have shifted increasingly to aggression previously linked with women, such as bullying others by excluding them from conversation, he said.

他说，日本男性渐渐地转向以前与女性有关的攻击，比如通过将人们排除在谈话之外来欺凌他人。

先前

1·His statement ran athwart what was previously said.

他讲的话与先前所说的相抵触。《牛津词典》

2·The contract was awarded to a previously unknown company.

该合同给了一家先前不知名的公司。《柯林斯英汉双解大词典》

3·I should like to add a corrective to what I have written previously.

我想对我先前写的内容进行补充修改。《牛津词典》

4·A previously unknown extremist group has said it carried out Friday's bomb attack.

一个先前未知的极端分子组织声称其实施了周五的炸弹袭击。《柯林斯英汉双解大词典》

5·Women's sports are breaking down the barriers in previously male-dominated domains.

女子体育运动正在打破障碍进入一些先前由男子占主导地位的领域。《柯林斯英汉双解大词典》

此前

1·The scientist confirmed the snake was a previously unknown species after conducting a genetic analysis of tissue samples.

在对组织样本进行基因分析后，科学家确认这条蛇是一个此前不为人知的物种。

2·Citizens of the world's wealthiest countries may live longer than previously predicted, according to a study sponsored by the National Institute on Aging.

美国国家老龄化研究所赞助的一项研究显示，世界最富裕国家的居民可能比此前预计的寿命更长。

3·The team was led by Dr. Graham Reynolds, from Harvard University, the scientist confirmed the snake was a previously unknown species after conducting a genetic analysis of tissue samples.

该研究小组由哈佛大学的格雷厄姆·雷诺兹博士带队，这位科学家在对组织样本进行基因分析后，确认这条蛇是一个此前不为人知的种类。

4·The team was led by Dr. Graham Reynolds, from Harvard University, the scientist confirmed the snake was a previously unknown species after conducting a genetic analysis of tissue samples.

该研究小组由哈佛大学的格雷厄姆·雷诺兹博士带队，这位科学家在对组织样本进行基因分析后，确认这条蛇是一个此前不为人知的物种。

5·A surge in new home sales and a drop in weekly unemployment claims suggest that the economy might not be as weak as some analysts previously thought.

新屋销售激增和每周失业救济人数下降表明，经济可能不像一些分析师此前预期的那样疲软。

过去

1·The effect was previously thought to be negligible, but researchers now say the shift will be large enough that it should be taken into account when interpreting how the Earth wobbles.

其影响在过去被认为是微不足道的，但现在研究人员表示，这一变化将大到足以让人们在解释地球摇晃的原因时必须把它考虑进去。

2·Labor markets which we previously thought were inexhaustible, like China and India, have actually tightened up quite dramatically.

中国、印度这些我们过去认为可能用之不竭的劳工市场，事实上已显著收紧。

3·But the big gap between the cost of building a smart-phone and its price in the shops should widen further as ever more previously discrete components are packed on to a single main microchip.

但是只能手机的制造成本和其在零售店里的价格的差距将会越来越大，因为过去很多分立的不见现在都能集成到一张主要芯片上了。

4·In fact, two of the factors that were previously used (goals scored and home advantage) will no longer have any impact on the ranking.

实际上，两个过去使用的因素(进球数和主场优势)将不再影响球队的排名。

5·This is a critical question because the young person who has previously attempted suicide is at higher risk of attempting it again compared with someone who never has.

这是一个关键性问题，因为过去有过自杀尝试的年轻人相比普通人有更高的风险再次试图自杀。

从前

1·The warmer temperatures mean that some of them can now grow peaches and maize where previously they could not.

气温升高意味着一些村民能种植桃子和玉米这些从前不能种植的作物了。

2·The actual error rate is 10 times greater than previously thought, despite a recent focus on reducing error rates and improving patient safety, a new study suggests.

实际发生的医疗差错数量是从前估计的10倍，尽管最近有研究报告，建议医院重视减少错误率，提高病人安全。

3·Previously its popularity has proved fleeting - will this time be any different?

从前它的流行总是转瞬即逝——这次会有什么不同吗？

4·Traub's analysis hints that there may be even more potential 'Earths' orbiting them than previously thought - and that this' principle 'would extend to stars not yet scanned by Kepler.

这项分析暗示，或许有比从前预想的更多的潜在“地球”在进行着轨道运动，这一原则将会拓展到还没被开普勒轨道天文台扫描到的地区。

5·But patients who get sick, including children, can return to school or work 24 hours after their fever breaks, instead of the full seven days previously recommended.

病人，包括儿童，如果患病，在他们发烧停止之后24小时就可以返校或是工作，代替从前推荐满七天。

早先

1·The game is known worldwide as Bang!, except in France where it was previously known as Wanted!

这款名为杀人的游戏在世界各地家喻户晓，但早先在法国它被称为通缉令。

2·Microsoft has previously paid out similar rewards to informants who helped identify the creator of Sasser, another notorious worm let loose in 2004.

微软早先向帮助确认震荡波制作者的人提供过相似的奖赏。震荡波是2004年爆发的另一臭名昭著的蠕虫病毒。

3·However, omega-3 blood levels in the fish oil group increased to a level previously found to have a positive cardiovascular effect, the researchers note.

然而，据研究人员所讲，服用鱼油人员欧米茄- 3血浓度上升至“早先发现的有益心血管”的水平。

4·A WHO collaborating laboratory in the United Kingdom has now confirmed 12 of the 21 cases of H5N1 avian influenza previously announced by the Turkish Ministry of Health.

世卫组织在英国的一个合作实验室现已确认土耳其卫生部早先宣布的21例H5N1禽流感中的12例。

5·Previously, Landerer and colleagues showed that global warming would cause Earth's mass to be redistributed towards higher latitudes.

早先，兰德雷尔和他的同事们研究发现，全球变暖会导致地球的质量向高纬度地区重新分配。

预先

1·A memory resource (sometimes called a memory pool) in the VM has grown to exceed a previously set threshold value.

虚拟机中的内存资源(有时叫做内存池)增长超过了预先设置的阈值。

2·The researchers studied DNA and hair colour information from hundreds of Europeans. They investigated genes previously known to influence the differences in hair colour.

研究者已调查了数以百计欧洲人的DNA及发色信息，并研究了预先知晓的决定发色的基因。

3·The size of a memory resource immediately after garbage collection is greater than a previously set threshold value.

垃圾收集之后 内存资源的大小超过了预先设置的阈值。

4·The consequence of the change was trivial and even beneficial: i.e., because we adjusted to this change, we had a more exciting interactive tour (than the landmark previously planned).

该变更的结果是微不足道，甚至是有益的：也就是，因为我们调整到此次变更，所以我们拥有了更加令人兴奋的交互旅行（比预先计划的里程碑）。

5·They used 65 respiratory specimens collected during April and May that had previously tested positive either for swine flu or one of the seasonal flu viruses.

他们应用在4月和5月收集的预先对猪流感或是季节流感病毒之一测试呈阳性的65个呼吸系样本。

在此之前

1·As pianos became more available, they brought classical music, the music which previously had been composed only for the upper classes, into the lives of the middle-class people as well.

随着钢琴越来越普及，古典音乐也随之进入了中产阶级的生活，而在此之前，古典音乐只是为上层阶级创作的。

2·Previously, it had awarded degrees from the University of London.

在此之前，该所大学的学位由伦敦大学颁发。

3·Previously, he said, Britain, the US and other Western countries had not been happy about the idea. Now they had changed their minds.

他说，在此之前，英国，美国以及其他一些西方国家对此想法很不“高兴”，但现在，他们已经改变主意了。

4·Previously, only the elite could afford to surround themselves with spells.

在此之前，只有上层才买得起咒符环绕其身。

5·Previously, the only blue roses available have been white ones which are dyed blue.

在此之前，唯一可用的蓝玫瑰是由白玫瑰染色而成的。