My teacher gave interesting assignments andmotivating the students.
很显然，这属于混用语法结构的一个典型错误。这会使句子产生歧义：难道是说My teacher gave interesting andmotivating assignments to the students? 因为本句中interesting和motivating 是平行结构. 或者是My teachergave interesting assignments and motivated thestudents? 因为亦可以理解成gave和 motivated是平行结构.所以错误使用语法会带来许多意想不到的麻烦。
当我们重复使用一个短语或单词的时候，不仅会给人以词汇量过小的感觉，有时甚至也会造成误解。我们来看一个例子：
My teacher wrote the assignment on the chalkboard. The assignment was on thechalkboard until the teacher erased the assignment after we had all done the assignment.事实上这句话的观点会更加清晰，如果我们将重复的词替换为其他表达的话。我们再来看看改进后的表达：
My teacher wrote the assignment on the chalkboard. She erased the board after we had allcompleted the task.
其中 assignment 被替换为 task; teacher 变为了she; chalkboard 被改为了board. 不但句子显得更加生动活泼不刻板，信息量也比原来要多了。
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