长句和短句是就句子的字数多少、形体长短而言的。长句和短句各有其优点和缺点。长句，因为使用的定语、状语较多，限制了概念的外延，增大了概念的内涵，所以比较精确、严密，但使用起来不够活泼简便。短句，由于字数少，直截了当，一般比较简洁、明快、有力，但不利于表达复杂的语义内容。长短句结构交替使用是雅思大作文的必备技巧，这既体现了节奏上的要求，也是意义上的需要。
例如：We can imagine the beautiful surroundings.There are many trees along the streets.There is a clean river in the city. There are many fishes in the river.There are willow trees on the one side.There are some pieces of grassland on the other side.There are many flowers on them.
文中七个句子都是简单句，句型结构单一，而且句子长短同一，都在七、八词左右，十分单调。
修改后的段落：Just imagine the beautiful surroundings if we make our cities greener.Green trees line the streets.A clean river winds through the city，in which a lot of fishes abound.On the one side stand rows of willow trees.On the other side lies a stretch of grassland sprinkled with many yellow and red flowers.
改写后的这段文字，有长句、、，也有短句和，一长一短，抑扬顿挫的节奏感就出来了。不仅句子长短交插，而且句型结构变化也很大，使文章流畅自然，生动活泼。