In English grammar, parallelism means using the same grammatical structure for the various phrases in a sentence.

Here is an example of a sentence that lacks parallelism:

     Jason plays football, lifts weights, and wrestling.

In this example, Jason is the subject of all three highlighted verbs. However, the last verb, wrestling, does not follow the same form as the first two, plays and lifts. To correct this, we will change the verb form of wrestling:

     Jason plays football, lifts weights, and wrestles.

Now this sentence is fully parallel.

The complement of a sentence sometimes might just have an adjective along with a modifying adverb. However, many students do tend to express this improperly:

     Jack very smart.

     This test quite difficult.

     The fight very violent.

Before the adjective, there always needs to be a verb like "be", "become", or "get":

     Jack is very smart. ("Is" is the conjugated form of "be".)

     This test gets quite difficult.

     The fight becomes very violent.

Now the above expressions are fully grammatical.

Sometimes, we might need to use a verb in the subject of a sentence. However, it is never proper to just use the verb in the base form when it appears in the subject:

     Run is very good for one's health. (Subject = "run")

     Do research is necessary for this assignment. (Subject = "do research")

     Go to school is something I do everyday. (Subject = "go to school")

Instead of using the base form, we need to use the gerund form. A gerund is a verb form that combines the present participle with the "ing" ending. If we change the highlighted verbs above into the gerund form, they would look as follows:

     Running is very good for one's health.

     Doing research is necessary for this assignment.

     Going to school is something I do everyday.

Now the sentences are grammatical!