Def.Parallel structure is a conjunction which is used to connect words or phrases that have the same grammatical function in a sentence.

Conjunctions: And, but, or, nor.
Those conjunctions are also called coordinate conjunctions.

For examples:

(a)Rina and Billy are having a meeting today.
(b)My brother and his English teacher went camping yesterday.
(c)He tried to tell the truth or (to) lie to us.
(d)The presentation is bad but reasonable.
(e)She walked to the door and seized the bag.


In (a) the conjunction /and/ connects noun /Rina/ and noun /Billy/. So, /and/ connect the same type of words.
In (b) the conjunction /and/ connects noun /my brother/ and a phrase /his English teacher/.
In (c) the conjunction /or/ connects infinitive /to tell/ and infinitive /(to) lie/. In this case, the type of infinitive is /to infinitive. Notice that the preposition /to/ here can be omitted.
In (d) the conjunction /but/ connects adjective /bad/ and adjective /reasonable/. And,
In (e) the conjunction /and/ connects verb /walked/ and verb /seized/.

From the explanation above we can make conclusion that:

NOTE: When the pattern is in progressive or continuous tense, The /be/ may be used or omitted.

For example:

He is watching TV and (is) doing his assignment.

Besides, A parallel structure can be more than two part of words (but the same grammatical function). Commas will be need to connect each of the words and ended by a conjunction /and/ or /or/.

For example:

- Winda, Tina, Shinta and Andri was reading some books in the library when Mr.Brown entered the room.
- The old man crept to the window, pushed it open and jumped into the water.

From Those examples above, we see that the parallel structure is placed in the subject. Besides, it can be placed in the object position.