Parallel structure

Items in a list or a comparison must have the same grammatical form: all nouns, all "-ing" words, all "to" verbs, all clauses.

How to do it

Find the list or the pair. Look for "and," "or," "both ... and," "not only ... but also," "either ... or."
Write the first item's form, then make every other item match it.
In comparisons, compare like with like: "the climate of England is milder than that of Russia," not "milder than Russia."

Example

Wrong Thoreau went to the woods to live simply, to read, and for thinking.

Right Thoreau went to the woods to live simply, to read, and to think.

The list is built of "to" verbs, so the last item is "to think."

Traps

"Not only" must be followed by "but also" in the same form: "not only in the city but also in the country."
A choice can be parallel but wordy. Once two choices match the form, take the shorter one.

Practice it

Every practice question comes with a full explanation, including why each wrong choice is wrong. Free, with no account.