Paying attention to the structure of your sentences helps to improve
your writing. One area where writers can make mistakes is with parallelism.
Parallelism means having balance within your sentences. For example,
a sentence that reads "He is responsible for calling the clients,
taking notes at meetings, and to read the reports" is not
parallel. To have a balanced, or parallel, sentence, it should read
"He is responsible for calling the clients, taking notes at the
meetings, and reading the reports."
A series of words like that in the sentence above always has to
be written in the same grammatical form--that is, if you have three
verbs ending in -ing, the fourth verb also has to have the same
ending. The rule is the same for sets of nouns or phrases used within
a sentence. For example, in the sentence "Students studying abroad
exhibit one or more of the following characteristics: happiness,
nervousness, or they are homesick," the characteristics listed are
not in parallel form. To make it balanced, the end would have to
read "happiness, nervousness, or homesickness."