In good English an attempt should be make to make the language as even and balanced as possible. this balance is called «parallel structure». You can achieve parallel structure by making the form of words as similar as possible. the following is an example of a sentence that is not parallel:
I like to sing and dancing.
The problem in this sentence is not the expression to sing, and the problem is not the word dancing. The expression to sing is correct by itself, and the word dancing is correct by itself. Both of the following sentences are correct:
I like to sing.
I like dancing.
The problem in the incorrect example is that to sing and dancing are joined together in one sentence with and. They are different forms where it is possible to have similar forms; therefore the example is not parallel. It can be corrected in two different ways: we can make the first expression like the second, or we can make the second expression like the first.
I like to sing and to dance.
I like singing and dancing.
There are several situations in which you should be particularly careful of parallel structure. Parallel structure are required in the following situations:
- With coordinate conjunctions, such as and, but, or.
- With paired conjunctions, such both…and, either…or, neither…nor, not only…but also.
- With comparisons.
In the next link you have some exercises for each skills (24-26):
QUIZ
TOEFL EXERCISE (Skills 24-26): In the questions with a space (____) choose the word or group of words that best completes the sentence and in the other questions choose the word or group of words that is not correct.
http://www.educaplay.com/es/actividades/1547254/actividad.swf?time=1411785641
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