It's a concept that is difficult to render into English.
这个概念难以用英语来表达。
Once a biological control agent is released, it is difficult to control its speed.
一旦实施了生物控制手段，它的速度就很难控制。
It is difficult to isolate the factors that led to the next development—the emergence of urban settlements.
推动其进一步发展，即城市聚居点出现的诸多因素，是很难孤立出来看待的。
The market situation is difficult to evaluate.
市场形势难以评估。
It is difficult to separate legend from truth.
传说与事实很难分辨。
The term 'mental illness' is difficult to define.
“精神病”这个词很难下定义。
It is difficult to define what makes him so popular.
很难解释清楚什么原因使他如此走红。
An unexplained death is difficult to come to terms with.
原因不明的死亡是难以接受的。
It is difficult to assess the full extent of the damage.
损失情况难以全面估计。
It is difficult to keep up with the rapid pace of change.
跟上迅速的变化是很困难的。
The cause of the disease is difficult to pin down precisely.
病因目前还难以解释清楚。
It is difficult to make oneself concentrate for long periods.
让自己长时间聚精会神是很困难的。
It is difficult to find work in the present economic climate.
在目前这种经济大气候下很难找到工作。
With so many variables, it is difficult to calculate the cost.
有这么多的可变因素，很难计算出成本。
It is difficult to understand the full import of this statement.
很难理解这份声明中的全部含意。
It is difficult to define the border between love and friendship.
爱情和友情之间的界线难以划清。
It is difficult to measure the success of the campaign at this stage.
在现阶段还难以估量这场运动的成败。
The seriousness of the situation is difficult to appreciate in its totality.
很难从全局上理解局势的严重性。
It is difficult to make a comparison with her previous book—they are completely different.
这很难与她以前的书相比—两者是截然不同的。
It is difficult to quantify an exact figure as firms are reluctant to declare their losses.
很难确定一个准确的数字，因为各家公司不愿公布它们的亏损情况。
English is difficult to learn at the beginning.
英语刚开始学起来很难。
In large cities, people complain it is difficult to breathe.
在大城市，人们抱怨呼吸困难。
These days it is difficult to imagine a world without books.
如今，很难想象一个没有书的世界。
It is difficult to build a dam in the sea, and the cost is very high.
在海里建水坝十分困难，成本也很高。
It is difficult to figure out a global population of polar bears as much of the range has been poorly studied.
由于大部分有关其生活范围的研究还很少，所以很难统计出全球北极能的数量。
It is difficult to attract young readers.
很难吸引年轻读者。
It is difficult to sound natural or plausible.
这很难听起来自然或可信。
It is difficult to teach in such a short time.
要在这么短的时间内教学很难。
In fact, it is difficult to buy a toy for a girl that is not pink.
事实上，给一个女孩买一个不是粉红色的玩具是很困难的。
When a city's population grows by 5% a year, it is difficult to keep up.
当一个城市的人口以每年5%的速度增长时，基础设施很难跟得上。
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