To gain a complete mastery of English, one must study very hard.

要想精通英语就必须刻苦努力。

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Conditions are changing , and to adapt oneself to the new conditions, one must study.

情况总是在不断变化，要想使自己适应新环境，就得学习。

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To understand the most important characteristics of a society, one must study its major cities.

要了解一个社会的最重要特征，人们必须研究它的主要城市。

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Because of the internal travel restrictions, people who want to move from one Palestinian city to another for work or study must register a change of address to make sure they can stay there.

ECONOMIST: The Palestinians

But commissioners did suggest that Congress, which must ultimately approve any changes, study the recommendations for at least one year before making a decision.

CNN: Panel proposes personal accounts for Social Security