两个句子之间不能只用逗号连接，要有连词。
误：My sister loves to dance，she is very good at it.
2.托福写作常见语法错误之句式不平行
句子中并列的成分应该是相同的结构。
误：I like to eat Japanese and eating Thai food.(eating-> eat)
误：Teenagers are more influenced by their peers than their parents.(than their parents-> than by their parents)
3.托福写作常见语法错误之句子不完整
句子缺少主语或谓语。
误：Because some necessary procedures such as transporting the factories，modifying equipment or planting more trees.
4.托福写作常见语法错误之句意重复或冗余
误：The low-priced car was inexpensive.(low-priced和inexpensive重复)
误：personally，I believe what the newspaper prints.(personally和I believe重复)
5.托福写作常见语法错误之时态，语态不一致
我们来看几种常见错误(句例说明)：
She did not adapt to the environment, but with time passing by, they find him more and more 和him是怎么个意思咧?
I shall use my father's experience as an example. My father went to Australia several years ago. He see many birds and animals.明明是过去，see怎么穿越回来了?
The gift I made are very pretty. gift是单数啊，are是怎么回事?
6.托福写作常见语法错误之连词类缺失
Successful men will have many places to live, a country house can provide a nice environment to relax, then it will be a new start point, but will not be an end one.
大家暂且忽略语法问题，且看这个句子。这个句子的第一个问题就是连词缺失：在live和a country house之间原作者使用逗号衔接的，但是呢，逗号并不能代替and等连词的.作用。因为Successful men will have many places to live和a country house can provide a nice environment to relax都是两个完整的句子，而没有主从依附关系，所以必须得有连词(and，but，therefore)等连接才能保证句子有效。
总结：
缺连词的另一大原因是很多同学不知道什么是连词，什么不是。and or but是最有名的三个连词。但是！however, so, yet, nevertheless也是连词！！！可以连接两句句子！