This is a small portion of the article, please go to my website to read the entire article at: Parallelism in Writing
As a rule, two or more sentence-elements that have the same logical office should be made grammatically parallel; i.e., if one is an infinitive, the other should be; if one is a relative clause, the other should be; if one is an appositive, the other should be; and so on.
Bad: The crowd began to wave handkerchiefs and shouting good-byes.
[“To wave” and “shouting,” both objects of “began,” are awkwardly dissimilar in form.]
Right: (a) The crowd began to wave handkerchiefs and to shout good-byes.
[or]
(b) The crowd began waving handkerchiefs and shouting good-byes.
[The two objects of ” began ” are made parallel; in (a) they are both infinitives, in (b) they are both gerunds.]
Bad: I met many people there whom I had seen before but did not know their names.
[“Whom I had seen before” and “did not know their names,” both qualifiers (logically) of “people,” are awkwardly dissimilar in form.]
Right: I met many people there whom I had seen before but whose names I did not know.
[The two qualifiers of “people” are made parallel; both are relative clauses.]