一个完整的句子有主语， 有动词，并且表达完整的意思。句子也被称作独立分句。一个片语尽管是以大写字母开头，标点符号结尾，还是不能称之为一个完整的句子，这是因为它不能表达完整的意思。它可能没有主语或者没有动词或者两者都没有。
下面就是两个片语
Where the mosquitoes made him miserable.
A way to learn about the city's history.
这两组片语皆不能独立成句。可以用两种方法进行修改。一种是使这些片语依附于一个完整的句子：
Simon left the campsite where the mosquitoes made him miserable.
另一种方法就是给这些片语加上主语、动词，或者其它词使其表达完整的意思：
The walking tour offered a way to learn about the city's history.
有时，读者可以通过前后句了解一个片语的意思。在修改这些片语时， 要做到不令读者费解。把片语改写成完整的句子，能更好的体现观点之间的衔接。
注意：有时候因为导入句后面遗漏了逗号，一个完整的句子可能会被看作一个片语。因此，当你以一个从属句作为句子的开头时，不要忘了在从属句后面点上逗号。
下面这个句子就是因为标点错误而被误认成片语：
错误句：If we have enough time this weekend we'll go to see the cherry trees in bloom.
在本句中"...we'll go to see the cherry trees in bloom" 是一个独立分句。"If we have enough time this weekend" 作为一个从句不是完整的句子。需要用一个逗号把引导性的从句和主句分开。
正确句：If we have enough time this weekend, we'll go to see the cherry trees in bloom.
A sentence is complete when it has a subject and a verb and expresses a complete thought. Another name for a sentence is an independent clause. A fragment is not a complete sentence because its thought is not complete, even though it starts with a capital letter and ends with a punctuation mark. It is missing either a subject or a verb or both.
Here are two examples of fragments:
Where the mosquitoes made him miserable.
A way to learn about the city's history.
These two groups of words cannot stand alone as complete sentences. They can be corrected in two ways. One way is to attach the fragment to a complete sentence:
Simon left the campsite where the mosquitoes made him miserable.
Another way to correct fragments is to add a complete subject, complete verb, or other words that express a complete thought:
The walking tour offered a way to learn about the city's history.
Sometimes readers can figure out the meaning of a fragment by rereading the sentences that come before and after it. As you revise, make sure that your reader does not have to struggle to understand your meaning. Turning fragments into complete sentences usually shows the connections between ideas better than a sentence fragment by itself can.
Note: Sometimes, a complete sentence may look like a fragment because it is missing a comma after an introductory word group. When you start a sentence with a dependent clause, be sure to use a comma after the dependent clause.
Here is an example of a sentence that looks like a fragment because it is incorrectly punctuated:
Incorrect: If we have enough time this weekend we'll go to see the cherry trees in bloom.
In this sentence, "...we'll go to see the cherry trees in bloom" is an independent clause that could stand alone. "If we have enough time this weekend" is a dependent clause that cannot stand alone. Use a comma to separate the introductory dependent clause from the main clause of the sentence:
Correct: If we have enough time this weekend, we'll go to see the cherry trees in bloom.