英语表达中有一个不成文的原则，叫作避免重复原则。比方说在并列句中，并列连词左右两边的部分，在内容上有重复的，一般后面的部分要省略掉重复的内容。但是两边部分的内容结构一定是对称的和平衡的。具体如下：
1.如果并列句中的左右两个分句主语虽然不同，但是两个分句有相同的be动词，那么可以省略连词和后一个分局的be动词
【例句】
Failing hips can be replaced, clinical depression controlled, cataracts removed in a 30-minute surgical procedure.
→Failing hips can be replaced,（省略and）clinical depression（省略can be）controlled,（省略and）cataracts（省略can be）removed in a 30-minute surgical procedure.
髋骨坏了可以更换，临床的忧郁症可以得到控制，白内障仅用30分钟手术就能治好。
2.如果并列句中左右两个分句出现重复的部分，那么后一个分句可以省略重复的部分，但是没重复的部分要保留
【例句】
Do you remember all those years when scientists argued that smoking would kill us but the doubters insisted that we did not know for sure?
→Do you remember all those years when scientists argued that smoking would kill us but the doubters insisted that we did not know（省略that smoking would kill us）for sure?
你还记得那些年科学家说吸烟会致命，但怀疑者坚持说我们并不确定吸烟是否真的会致命吗？
3.在并列句中后边的分句可以省略与前边分句中相同的成分
【例句】
Tom picked up a coin in the road and handed it to a policeman.
→Tom picked up a coin in the road and（省略Tom）handed it to a policeman.
汤姆在马路上拾起一枚硬币并把它交给了警察。
补充知识点：
并列连词and, but, as well as, or, or else, both…and…, neither…nor, either…or, not only…but (also), rather than, as…as等以及从属连词than可连接两个对等的词和对等的结构。
（1）所连接的谓语形式必须一致
【例句】He went shopping, bought some gifts and visited his old friend.
（2）所连接的词或短语形式必须一致
【例句】We often go fishing as well as swimming.
（3）连接的非谓语动词的形式必须一致
【例句】People find eating vegetables more healthy than fasting.
（4）所连接的句子结构必须一致
【例句】My girlfriend is pleased with what I have said and all that I have done.