Items in a Series
One of the most common parallel structure mistakes occurs when writers list items in a series. You're bounding over the hills and dales of a sentence when suddenly, just before you reach your destination, the car hits a ditch. Here's an example:
Sasquatch enjoys taking long walks in the forest, playing with small woodland creatures, and to devour wandering tourists.
Now, instead of all -ing words - called gerunds - in the series, the sentence hits a hitch at the infinitive to devour. Matching the verb-forms restores the balance, so it should be:
Sasquatch enjoys taking long walks in the forest, playing with small woodland creatures, and devouring wandering tourists.
Isn't that better?
Clauses
In general, if you begin with a clause (that's a group of related words that contains a subject and a verb), you need to stick with that clause for the duration of the sentence to keep it parallel. So try this:
On the last night of awesome ninja training, the Master told his students that they must be stealthy, that they must be cunning, and to be quick.
This sentence is unbalanced because the infinitive to be quick is not parallel with the clause that they must be. To fix this, replace to be quick with the same clause as the other clauses in the sentence:
On the last night of awesome ninja training, the Master told his students that they must be stealthy, that they must be cunning, and that they must be quick.
Or, to make it more graceful - but still parallel - in this instance, you can just list the final adjectives of the clause:
On the last night of awesome ninja training, the Master told his students that they must be stealthy, cunning, and quick. That's ninja-esque.
Comparison Sentences
Comparison sentences can also be parallel or not parallel. For instance:
I like to time travel more than flying.
The two comparison elements here (to time travel and flying) aren't in the same grammatical form. To time travelis in the infinitive form, while flying is a gerund. To make the sentence parallel, use the gerund in both cases (those are those -ing words we talked about before).
I like time traveling more than flying.
Or you can use the infinitive form in both cases:
I like to time travel more than I like to fly.
Either way, the sentence is now balanced.
Verb Tenses
When writing parallel sentences, you also want to make sure your verb tenses are the same. As before, remember to look on either side of the conjunction or connecting phrase to make sure the forms match.
So, Jonathan had grabbed the stake and drove it into the vampire's heart is not parallel because had grabbed on the one side of the conjunction (and) is in the past perfect form, while driving is simply in the past tense. Balance the verb forms to make the sentence parallel. Can you see it?
You should choose either Jonathan had grabbed the stake and driven it into the vampire's heart, or Jonathan grabbed the stake and drove it into the vampire's heart. Either one is correct.