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。
多次强调过，在说口语时千万要注意时态、人称和数量的统一，而这些oral slips却是中国学生最容易出现的问题。同时，这些问题会让你的听众们糊涂，不明白你究竟要表达哪种意思。看一个例子：
My teacher brought five paper bags to school one day. He put us into groups and gave each group a bag. You have to take the objects out of the bags in turn and then a person has to tell a story involving the object from the bag.
前面是一般过去时，后面突然转为现在时;同时人称也由“我们”变为“你”最后是一个“他”。最后单复的变化亦会令人很费解。估计考官们很难跟得上这种跳跃的思维，所以保持前后一致性很重要。我们来看改后的版本：
One day my teacher put us into five different groups. He gave each group a bag and told us to take turns pulling out an object and telling the other members of the group a story involving that object.