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。
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 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。
It is difficult to train pigeons to escape from a certain situation by pecking.
训练鸽子通过叼啄来脱离某种情境是很困难的。
With such a long interval, it is difficult to determine the age of this fossil.
时间隔得这么久了，很难确定这化石的年代。
Sometimes the instructions are very simple but the furniture itself is difficult to build.
有时候说明书很简单，但是家具本身很难做。
The contribution of geothermal energy to the world's energy future is difficult to estimate.
地热能对世界未来能源的贡献是难以估计的。
It is difficult to conceive of vigorous economic growth without an efficient transport system.
如果没有高效的交通系统，很难想象会有强劲的经济增长。
It is difficult to engrave in marble.
大理石很难雕刻。
It is difficult to obtain reliable evidence.
很难获得可靠的证据。
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