Short Explanation:

Parallel structure refers to words having the same grammatical structure, for example, matching suffixes or verb conjugations. Parallel structure is often used when words or phrases are listed in a series (and all items in the series are of equal importance) or when words and phrases are being compared directly.

• Lists often include the linking words and or or.
• Comparisons often include the linking words and, but, than, or or

Examples:

I like apples, grapes, and bananas.
Apples, grapes, and bananas are all nouns listed in their generic plural forms.

Swimming, dancing, and writing letters to my friends are some of my hobbies.
Swimming, dancing, and writing are all listed in the gerund (-ing) form.

Its novelty, modern design, and reasonable cost have helped make the new computer popular amongst the university's freshmen students.
Novelty, design, and cost are reasons listed in their noun forms.

The mixture must be thoroughly combined, poured carefully into the appropriate molds, dried for at least 12 hours, and finally released from its molds without being broken before it is sold.
Combined, poured, dried, and released are all verbs listed in the past tense that describe steps in a process.

When dining with friends or with relatives, always be sure to be extra polite.
The word friends is given in the plural form, so the word it is compared to (i.e., relatives) should also be in the plural form.

Examples in the Official Guide to the TOEFL ITP® Assessment Series:

• Page 31: Example 2
• Page 31: Number 10
• Page 74: Number 23
• Page 75: Number 29
• Page 75: Number 34
• Page 116: Number 23
• Page 117: Number 25
• Page 118: Number 37

Structure Style Practice Questions:

Written Expression Style Practice Questions: