文章是通过一个一个小篇章来表达思想和主题的，篇章的构造需要词与句子作为它的基本原料，我们说如果说文章的主题思想是灵魂的话，那句子就是文章无数的血管。血液不通畅文章也就将难成好的文章，我们在对付PETS考试中的作文时，一定要拿捏好以下几个改句、造句的原则：
(一)主题句原则
一个好的主题句是文章成功的一半。不仅文章有主题，段落有纲，句也必有中心。文章失却主题，就会不知所云;落无纲，则一盘散沙，句缺中心，就无意义。所以，在立足于篇章时，一定要首先明确文的主题;在发展段落时，也要首先写好主题句。从PETS三的特点来考虑，主题句最好是置于文首或段首，这样让人一目了然! 以下就是两个实用的开门见山式发展主题句的方法。
●To begin with, we must attach the greatest importance possible to this issue/fact (key word).
●As is well known to all, it is now high time that we took this (topic word) into serious consideration.
(二) 句式变化原则
好的文章动静结合、张弛有度有节奏感。句式也是一样，要有变化性，这不仅能使文章更生动，也是语言表达方式的需要和表达能力的体现。句式的变化，主要是要注意两点：
● 不要从头至尾使用一种句型。
● 长短句结合。
由于语言功底的欠缺和惯用思维，很多人写的文章一种句式到头，如：
I think …
I hope…
He does it.
He will take it
这样的文章虽然意思表达出来了，却显得呆板，欠生动。解决的办法是：
1.间或使用主从复句。
Because he is very much determined, he will carry it out this time. (原因)
比较：He is very much determined. He will carry it out this time.
2.使用分词结构句。
The weather being fine, a large number of people went sightseeing.
比较：The weather is fine. A large number of people went sightseeing.
Africa is the second largest continent, its size being about three times that of China.
比较：Africa is the second largest continent. Its size is about three times that of China.
3.使用不定式句。
To be or not to be, that is a question. (莎士比亚)
To study or not to study, that is much different.(引申)
To do it well, you must plan it well.
比较：You want to do it well. And you must plan it well.
4.倒装句
Only when we fully recognize its importance can we have control of its essence.
比较：After we fully recognize its importance, we can have control of its essence.
No sooner had he arrived home than it began to rain.
比较：He arrived home. And it began to rain.
5.失衡句
whether or not he will come is still unknown to all the people present.
比较：No one present knows whether he will come or not.
That he has done it all by himself is known to everyone.
比较：Everyone knows that he has done it all by himself.
当然，句子并不是越长越好，也不是越复杂水平就越高。凡事皆有度。太多长句的堆砌让人觉得是买弄。有时侯，一个短小精辟的句子可以起到画龙点睛的作用。特别是在文末段尾。比如：
●As a creature, every one knows.
●Actions speak louder than words.
●Practice makes perfect.