The rule goes that in a sentence with pairs of thing or a list of things, those things must be in the same grammatical format as each other. 

It might be easier to explain by demonstrating.  If you are listing possible answers to a question, you might say, "1, 2, and 3."  Or "a, b, and c."  But "1, 2, and c," doesn't work.  That's what happens when you write sentences like When he returned from the hunting trip, he was tired, hungry, and he had a sore on his heel.

Look at it this way.  There's the stem -- that is, the core part of the sentence.  Every part in the "list" must connect correctly to that stem.  The commas separate the elements of the list. 

 
 

STEM

list

When he returned from the hunting trip, he was tired
When he returned from the hunting trip, he was hungry
When he returned from the hunting trip, he was he had a sore on his heel  
 
  (Note:  When you try this out, matching each element to the stem, you need to ignore the combining word -- the ors, the ands, or the buts.  The only thing those words help with is knowing where the last element starts and how the elements relate to each other.)  
Stem + list

For some reason, after the first elements of the list, people seem to just let loose with the last element, turning it into something quite different from the other elements.  (The last element isn't always the problem, but it is most commonly the problem, perhaps because at that point we lose sight of the original stem.)

  • Heavy metal poison remains in the body, the liver, the hair, and provides a timeline of the poisonings.
Heavy metal poison remains in the body,
Heavy metal poison remains in the liver,
Heavy metal poison remains in the hair,
Heavy metal poison remains in provides a timeline of the poisonings.     

 

 
Stem + list +
stem
  • At the grocery store, we bought bottled water, turkey ham, and we found a neat little blowtorch for the camping trip.  (In this sentence, we have two parts for the stem, before and after the list.)
At the grocery store, we bought bottled water for the camping trip
At the grocery store, we bought turkey ham for the camping trip
At the grocery store, we bought we found a neat little blowtorch for the camping trip  

 

 
 
  • Because Ellen was late to work so many days, she didn't seem to care, and her frequent mistakes, her boss fired her.
Because Ellen was late to work so many days her boss fired her.
Because she didn't seem to care her boss fired her.
Because her frequent mistakes her boss fired her.   

 

 
 
  • I needed to fill out the worksheet, study my notes, and we were going to get together to practice before the chemistry test. 
I needed to fill out the worksheet, before the chemistry test
I needed to study my notes before the chemistry test
I needed to we were going to get together to practice before the chemistry test   

 

 

 

 

This rule is true even for lists of two parts.  Can you see what's wrong with the following sentences?

  • The weather rained and snowy. 
  • He might have driven or walking back from the office.
  • We do that by adding acid to the solid sample and digests it.
     
The weather rained
The weather snowy    

 

He might have driven back from the office.
He might have walking  back from the office.    

 

We do that by adding acid to the solid sample
We do that by digests it   

 

 
Paired connectors

We see more examples in phrases introducing pairs:  either. . .or, neither . . .nor, both . . . and, not only. . . but also.

She was not only intelligent, but also she had been highly educated.

She was (not only) intelligent
She was (but also) she had been highly educated   

They either forgot the appointment, or they got lost.

They (either) forgot the appointment
They (or) they got lost.   

 

 
  The solution to parallel problems is to turn all the elements of the lists into the same format.  If that can't be done, don't write it as a list.  If, for example, the third element doesn't fit and you can't make it fit, then finish the list with the second element and write the third one into a new sentence.  The little orange ball is a link to possible corrections for each of the sentences above.  Try correcting the sentences yourself, then click on the links and see what I provide.  
  This page last updated by Connie Gulick, Tuesday, May 18, 2010.