Hi steves5a,

If you mean “Parallel Structures” by “Parallel parts”, I can explain them  as in the below;

Some conjunctions combine with other words to form what are called correlative conjunctions. These are

both . . . and
not only . . . but also
not . . . but
either . . . or

neither . . . nor
whether . . . or
as . . . as

They always travel in pairs, joining various sentence elements that should be treated as grammatically equal.

-She led the team not only in statistics but also by virtue of her enthusiasm.

-Polonius said, "Neither a borrower nor a lender be."

-Whether you win this race or lose it doesn't matter as long as you do your best

Correlative expressions should be followed by the same grammatical construction. Many violations of this rule can be corrected by rearranging the sentence.

-It was both a long ceremony and very tedious. F

The ceremony was both long and tedious. T

-A time not for words, but action F

A time not for words, but for action T

-Either you must grant his request or incur his ill will. F

You must either grant his request or incur his ill will.  T

My objections are, first, the injustice of the measure; second, that it is unconstitutional. F

My objections are, first, that the measure is unjust; second, that it is unconstitutional. T

Hope this helps

Sure it should be "The seminar speaker's talk was motivating and challenged."

By the way I feel structural parallelism is not a matter of grammar but the matter of style. I remember I was pointed out by Khoff that a sentence like "I'm a Japanese student and learning English in the university extension" sounds natural to native speakers despite the fact that the sentence is against the structural parallelism.

paco

I found in another English learners' site a post like below. The original sentence was "The seminar speaker's talk was motivating and challenged us". This sentence is not in a parallel construct, but I don't think it is ungrammatical.  What is your opinion? 

Which sentence contains parallel parts? a) In the business world, you must write clearly and with conciseness. b) I would rather drive a car than ride a bike to work. c) Joan is a member of the internal audit committee, a team ;eader, and plays on the company softball team. d) The seminar speaker's talk was motivating and challenged us. My answer d) ?

paco