When two or more things are parallel, they have a similar function, role or structure.
In grammar, “parallel structure” refers to the balanced construction of a sentence.
A balanced sentence will present the items in a series by using the same grammatical form for each item in the series.
For example, if the first item in a list is expressed as a single word, the remaining items will also be expressed that way:
My favorite flowers are pansies, daffodils, and roses.
The children were noisy, muddy, and happy.
If the first item in the list is expressed as a phrase, the remaining items will continue according to the same pattern:
I like to read, to write, and to garden.
We looked under the bed, in the closet, and behind the sofa.
The candidate’s immediate goals are raising money, charming voters, and winning the election.
If the first item in the series is a clause, the remaining items will also be clauses:
She is looking for a man who likes children, who tolerates uncertainty, and who loves his mother.
“Faulty parallelism” occurs when the series is made up of grammatical forms that do not match. Here are examples of faulty parallelism with corrections:
Faulty: The crowd outside the embassy was loud, boisterous, and they were angry.
Balanced: The crowd outside the embassy was loud, boisterous, and angry.
Faulty: Now that he’s won the lottery, he is selling the house, plans to move to Italy, and will take the family with him.
Balanced: Now that he’s won the lottery, he is selling the house, moving to Italy, and taking the family with him.
Faulty: In staffing the new school, we looked for teachers whose work was innovative, with solid academic credentials, and who could speak standard English.
Balanced: In staffing the new school, we looked for teachers whose work was innovative, whose academic credentials were solid, and whose English was standard.
I find these non-parallel construction frequently too! It seems to me that the authors mean to join two clauses but forgot or neglected to join them properly.
For ‘The crowd outside the embassy was loud, boisterous, and they were angry’
I read ‘The crowd outside the embassy was loud and boisterous, and [it was] angry.’
Likewise, ‘Now that he’s won the lottery, he is selling the house, plans to move to Italy, and will take the family with him.’
I read, ‘Now that he’s won the lottery, he is selling the house and plans to move to Italy, and will take the family with him.’
I feel the rule that ‘and’ is used only before the last item in a series is taken too far here, whereas ‘and’ is fine if its joining two separate series.