According to Grammarly, this is the definition of parallelism:

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Parallelism in grammar is defined as two or more phrases or clauses in a sentence that have the same grammatical structure.

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Parallelism, in syntax, is a way to structure your sentences harmoniously. Utilizing a parallel structure in your sentences in a text you write improves your writing style and makes your composition more elegant. Let's analyze the following example:

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Research estimates indicate that people who like reading, exercising, and to talk to friends tend to have better mental health.

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What is wrong with this sentence?

Grammatically, it is not technically incorrect. However, it is an unbalanced sentence. Sentences like this can make your writing unsophisticated. In this case, the issue was the verb form, which should be the same from beginning to end. Notice that the gerund (-ing form of the verb) is used as well as the infinitive (the form of the verb which accompanies the particle to-). So as to correct the sentence, we can write it in two ways:

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Research estimates indicate that people who like reading, exercising, and talking to friends tend to have better mental health.

OR

Research estimates indicate that people who like to read, to exercise, and to talk to friends tend to have better mental health. (the particles in red could be omitted)

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In a list, for example, it is important not to change the grammatical form of the items listed. Below is a checklist of how to verify the balance of a sentence (if it has a parallel structure):

1- nouns with nouns

• singular with singular

• plural with plural

2- verbs with verbs

• tense X with tense X

• gerund with gerund

• infinitive with infinitive

3- adjectives with adjectives

4- adverbs with adverbs

5- phrases with phrases

6- clauses with clauses

How about we correct a few mistakes concerning the parallelism of the sentences below?

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She likes volleyball and playing videogames.

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Do you see what is wrong with this sentence?

In it, there are two nouns: "volleyball" and "videogames". However, the verb "playing" destabilizes the sentence. How do we reestablish its balance? There are two ways to do that:

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She likes volleyball and videogames. (noun with noun)

OR

She likes playing volleyball and playing videogames. (verb with verb, but the one in red could be omitted)

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Now, the sentence has balance. Let's do a few more, shall we?

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He wrote his text cohesively and with coherence.

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If you focus on how this sentence presents how he wrote his text, you will notice that it says it was written "cohesively" to afterwards say "with coherence". "Cohesively" is an adverb of manner, but is "with coherence"? Not really. In spite of acting as one, it is actually a prepositional phrase. How do we change that?

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He wrote his text cohesively and coherently. (adverb with adverb)

OR

He wrote his text with cohesion and with coherence. (prepositional phrase with prepositional phrase, but the preposition in red could be omitted)

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Notice that the sentences have balance in their structures after the modifications made. Let's do another exercise.

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Reading is beneficial for both children and for adults.

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Reading that sentence is a little tough, right? That could even be the topic sentence of one of the paragraphs of an essay. Opening a paragraph with a sentence which sounds so unstylish would undoubtedly not make a good impression on the reader. How would you rewrite this sentence so as to make it classier? There are two possible answers:

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Reading is beneficial for both children and adults.

OR

Reading is beneficial both for children and for adults.

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Now, the sentence sounds a lot better. The example below is similar to this one. Do it to then see the rule which applies to these.

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Not only are puzzles entertaining, but they also have advantages.

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The word "entertaining" is an adjective, but "they also have advantages" is a whole clause! Fear not, for there is a simple solution. The word "advantage" is a noun. What is its corresponding adjective? If you answered "advantageous", you are right on the money! Now, let's make the proper alterations to the sentence using the rules of parallelism by adapting its syntax.

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Not only are puzzles entertaining, but also advantageous. (adjective with adjective)

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Here is one noteworthy detail: when you use structures like "both... and" and "not only..., but also", you should pay extra attention to parallelism. Here is a list of those five structures:

1. both ❌ and 🔵;

2. not ❌, but 🔵;

3. not only ❌, but also 🔵;

4. neither ❌ nor 🔵;

5. either ❌ or 🔵.

❌ and 🔵 belong to the same grammatical classification.

Below, you will find two SAT questions. Apply the knowledge you have acquired here to answer them. I will also leave the link of the website from which I got them so that you can check out the complete explanation for the answers.