In parallel structures or parallelism different parts of a sentence should be in symmetry. The symmetrical order is maintained in a sentence by using the same verb form, same tense and other parts of speech should also be same, like noun or adjective.
Consider the sentences below:
- I like to play guitar and singing. (Incorrect)
I like playing guitar and singing. (Correct)
- Mansi likes to watch televisition, ice-creams and carrom. (Incorrect)
Mansi likes to watch television, eat ice creams and play carrom. (Correct)
- The idea is fantastic but a danger. (Incorrect)
The idea is fantastic but dangerous. (Correct)
The concept of parallel structures is very important in the english usage section and sometimes two to three options can be eliminated because they do not follow a parallel structure in a sentence. If you see a window in a room of which one pane is painted and the other is not. This will look non-symmetrical and to make it look symmetrical you would want to paint the other pane also. This is exactly what parallelism in language does, it makes the structure symmetrical!