My teacher gave interesting assignments and motivating 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 and motivating assignments to the students? 因为本句中 interesting和motivating 是平行结构.

或者是My teacher gave interesting assignments and motivated the students? 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 the chalkboard 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 all completed 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。